Looping Lines

This is where an actor is called back to do the lines again, usually because something went wrong with the sound recording on-set. Also known as "automated dialog replacement" (ADR).

The fun part? Now the actor gets to do the line with the intended emotions while lip-synching themselves.

This is much more common than you may think. As a rule of thumb, any time you hear an actor make a noise and you can't clearly see their mouth on screen, it's been ADR'd. This technique is used on a wider and more noticeable scale in Asian live-action productions, particularly Bollywood and Japanese Tokusatsu and dorama television serials. Voice acting in anime is also like in North America (though in Japan, this is reverse where anime is pre-lay while Western Animation is ADR).

This is the standard way to record dialog in dubbed foreign productions and in animation, when the lines aren't recorded beforehand.

Film

 * In Star Wars: Episode 3, Hayden Christensen and Ewan Mcgregor had to record grunts for when they were fighting. That's right. Grunts.
 * This is standard procedure for fight scenes, such sounds made on-set are drowned out by the sounds of the fight.
 * Looping was a fact of life for many actors in the Star Wars universe, like Anthony Daniels (Threepio). Most well-known case is Darth Vader, as no one would be afraid of David Prowse's voice.
 * Very noticeable in A New Hope as dialogue tends to fluctuate in volume and intonation even in the same scene.
 * The Lord of the Rings--the entire trilogy--was dubbed in post: the enormous noise of the fans and the assorted background noise made it impossible to hear any of the dialogue. That is, the whirling bladed fans, not the squeeing or complaining ones.
 * In the pre-Internet, pre-videotape, pre-digital era of actual filmed pornography, sex scenes were typically filmed without sound; performers would then add all the various grunts, groans and exclamations afterward. Most of the time this would be done haphazardly, with only minimal care for any visible Mouth Flaps; this often added an unintentional Hong Kong Dub quality to the scenes.
 * Still the case in quite a lot of digitally shot porn. Sadly, in most of the rest of the cases, decent audio normalization and levelling would be a huge help. Sigh.
 * This is hilariously referenced in Jesus Of Montreal where one of the actors is introduced while looping lines for a porno. Halfway through, he cries out that he's been reading the wrong lines and he's told that it doesn't matter and that no one will notice.
 * This trope is often completely averted with "gonzo" and "reality" porn, where all the live audio is left in, up to and including "actor" directions. There's a bit of a Broken Base as to whether or not this adds positives to the scenes (with the director acting as something of a narrartor) or renders said scenes unwatchable with the sound on.
 * Hugo Weaving as V in V for Vendetta had to dub all of his lines, because of the mask.
 * Also because V was recast. He was originally played by James Purefoy, who left a few weeks into filming.
 * In Dr. Strangelove, there is a noticable scene where Major Kong is saying "Dallas" but you hear "Vegas", in order to prevent a Funny Aneurysm Moment based on the recent assassination of JFK in Dallas.
 * The "chompers" scene in Galaxy Quest features Sigourney Weaver's character seeing the completely nonsensical hallway full of banging metal blocks and exclaiming, "Screw that!"--except that from the movement of her mouth it's entirely clear that she originally said "fuck". Presumably the line was looped to keep the film to a PG rating. There are also a couple of other lines in the film that don't disguise the dub as well.
 * The plot of Singin in The Rain (such as it is) centers around this.
 * This was standard practice in many Italian movies until about the '80s. Films with multilingual casts (such as many Spaghetti Westerns) were often shot without any microphones on set, and with each actor saying his lines in his own language. Hence, these films do not have one original or "official" language track; every version is a dub.
 * A good chunk of the dialogue from The Descent had to be dubbed in, because the sets were polystyrene and sure didn't sound like a cave or rock while they were walking or moving around on it.
 * Cillian Murphy rerecorded all of his lines for Twenty Eight Days Later during Post Production, replacing a faux-British accent with his natural Irish accent because he thought the British accent sounded too fake.
 * There was quite a bit of ADR going on, according to Danny Boyle. Several shots were actually set up to accommodate ease of ADR (faces in shadows, the actor out of frame etc) and a handful of new lines were added to otherwise wordless scenes.
 * An infamous goof in Eegah was a result of the botching of one of these: at one point, while the main characters are walking, one of them suddenly "shouts" "Watch out for snakes!" despite the fact that the character isn't speaking and the quality of the line's audio not matching the rest of the movie. As such, "Watch out for snakes!" has become a Running Gag on Mystery Science Theater 3000, (where Eegah was shown) and went through a bit of Memetic Mutation as well.
 * The scene in Love Actually where Aurelia removes the cup holding down a portion of the book Jamie is working on had to be redubbed due to the noise of the large fan sitting off camera.
 * Thunderball, the fourth James Bond film, shows Q introducing Bond to his new tricked-out briefcase, accompanied by the line "Now pay attention, 007". This line would later become one of many famous motifs in the series, but Desmond Llewelyn does not move his mouth.
 * In Goldfinger, Gert Fröbe's lines had to be dubbed over because of his impenetrable German accent.
 * One actress, Nikki Van Der Zyl, was the voice of the Bond Girl twice (Dr. No and Thunderball), the Bond Girl in From Russia With Love was also dubbed, and Robert Rietty voiced both an agent who is killed in Dr. No and the villain of Thunderball.
 * The Bond films in general rely heavily on ADR for the same reason as the The Lord of the Rings trilogy.
 * In Hot Fuzz, Simon Pegg redubbed most of his lines from the second half of the movie to make his voice sound more like an 80s action hero.
 * A visible example shows up in The Godfather: Sollozzo learns that Don Vito Corleone is still alive after the assassination attempt he ordered, and says to hostage Tom Hagen "That's bad luck for me, and bad luck for you if you don't make that deal!" before apparently releasing him. However, if you look closely you see that Sollozzo just says "That's bad luck for me, and bad luck for you": there was a short scene that was present in the book but cut from the film, where Tom Hagen arrives back at home and exclaims "Boy, if I argue against the Supreme Court I'll never do better than I did against that Turk tonight!", having convinced Sollozzo not to kill him on the grounds that he could still negotiate a deal with Sonny despite the Don being alive.
 * Evil Dead 2 has Ash, after his hand has been possessed, screaming and running around, then saying very distinctly "Work shed." Those two words were looped in ADR, and it sounds like it. In fact, Bruce Campbell relates on the commentary that years later, when he met Kurt Russell for the first time, Russell walked up to him, shook hands, and without preamble demanded Campbell "say 'Work shed'."
 * According to the post-production supervisor, the Super Mario Bros. movie had the most ADR-looping of any film she had ever encountered.
 * Averted in the Assembly Cut of Alien 3. Some of the restored scenes were cut before the ADR was recorded, and since they didn't do any re-recording for the DVD, it can be difficult to hear the dialogue. Thankfully, subtitles are avaliable.
 * Quite a bit of dubbing was going on in the last half of Dazed and Confused. It's not terribly noticeable except when one character is visibly speaking (and gesturing dramatically) and you're hearing a different character's voice. Then it's odd.
 * Happens all the time with Tommy Wiseau's lines in The Room, for no adequately explained reason.
 * Lina Leandersson in Let the Right One In was dubbed over by an older actress since the director found her voice to be too feminine to suit Eli.
 * In the 1979 version of Dracula, the dialogue just before Dracula sucks Lucy's blood had to be looped because the dramatic fog machines made too much noise.
 * During the opening of Hell Up In Harlem, lead character Tommy Gibbs is sitting in a taxicab that's being pursued by two mob henchmen. At several points, Gibbs can be heard saying lines (such as "Run that red light!", "Step on it, man! They got guns!" and "Here's $500 - don't stop for anything!") that don't match up to his mouth. This is due to the fact that Harlem rewrites the ending of Black Caesar with different dialogue placed in existing scenes.

Live Action TV

 * Anthony Stewart Head had to loop many of his lines as Giles on Buffy the Vampire Slayer in order to incorporate the character's mild stutter. This was such a hassle that Head declared he would never play a character with a speech impediment again.
 * The Babylon Five episode Comes the Inquisitor was originally broadcast with Sheridan referring to the historical murders in London's West End. Which is fine, I'm sure there were as many murders there as in the rest of the city... but they meant Jack the Ripper, who committed murders in London's East End. The error was pointed out in a usenet post and corrected for the subsequent broadcast. Not very well, as they hadn't intended to loop it and the camera was right on his face for the whole scene. Watch his mouth, and it's very obvious he says "West" when the audio says "East".
 * J Michael Straczynski originally said that despite the mistake, he didn't want to change the dialouge because a) the camera was right on Sheridan's face, and b) it would sound different from all the other dialogue. He probably got tired of hearing from fans telling him he got it wrong.
 * On Jeopardy!, Alex Trebek often rerecords his reading of a clue if he stumbles over a word, but no lip-synching is involved because the screen usually shows the text of the clue rather than the host's face.
 * Lost requires a lot of looping because almost all of the action is filmed outdoors in Hawaii. The ambient noise at times covers up the dialogue.
 * It has been suggested that Donald Trump loops his boardroom speeches on The Apprentice. This may be to complete the editing "story" about why someone is getting fired, or simply to insert something more eloquent than what was actually said.
 * This is the standard method of recording lines in Super Sentai despite the fact that it's Live Action TV: the actors act out their scenes in front of the camera and then re-record every single line in a recording studio. According to one director, this is by far the hardest part of production, and also explains why most characters end up in a Milking the Giant Cow situation.
 * Used for comic effect in Garth Marenghi's Darkplace - often lines either don't fit the Mouth Flaps, or characters yell random lines of expositions without moving their lips.
 * The Made for TV Movie Kiss Meets the Phantom of the Park was heavily looped. This led to Peter Criss, already fed up with the filming and his bandmates, refusing to loop his lines -- so all his dialogue was looped by a voice actor. This is hardly the only problem with the movie, but it's by far the most glaringly obvious.
 * The early Star Trek the Next Generation episode The Battle featured the appearance of Picard's old ship the Stargazer. The show was shot to the original script before the model shots of the ship were filmed, which had intended to use the old movie Enterprise model to represent it, and Laforge names it as a Constitution-class ship. However, the producers then changed their mind and made up a new model representing a previously unseen class. Levar Burton then redubbed his line using a similar-sounding but different class name, Constellation.
 * Very rarely a line is dubbed over quickly days or hours before broadcast of a series to remove a line in the wake of a tragic event where a punchline which was fine days before would now go over like a lead balloon, as in when Lauren Graham redid a Gilmore Girls one-liner punchline about Bali to instead be Maui days after the Bali nightclub bombings. However the Closed Captioning track had already been laid down (which is much, much more complicated to fix, requiring almost a complete re-do of an entire scene), so viewers with it on still saw the Bali reference.
 * There's a point in a Scrubs episode where Dr. Cox is yelling at Elliot (that narrows it down a bunch...) and begs to 'Aiisha', even though it's very, VERY obvious for anyone watching his mouth that he's saying "Allah".
 * On TV shows like Glee and Victorious where the cast is singing, they will lip sync to a prerecorded track of themselves singing so the audio quality is better and so they can concentrate on dancing instead of singing and dancing at the same time.
 * The Doctor Who episode 'Midnight' had a lot of ADR. So much so that the corresponding Doctor Who Confidential episode was pretty much entirely about the ADR process.
 * The looping in of lines in Burn Notice isn't very smoothly done, and there are strong changes in ambient noise and acoustics that are sometimes overly noticeable.
 * A lot of reality shows include ADR-looping to explain the rules over and over each episode, for the audience's sake. This is very noticable in shows like Project Runway and Top Chef.

Web Original

 * Dr. Horrible. Everyone singing is their own voice, but it's a dub of the official musical track over the scene. So they're essentially lipsynching to themselves.
 * This practice is near-universal for filmed musicals, due to the difficulties of having a full orchestra (or even a partial one) on set during filming, of coordinating an offstage band to an on-camera singer (who can't be looking at the director), and of having to do multiple takes - more people involved means more chance for error, and when even one screw-up requires a do-over, it's far, far easier to simply prerecord the tricky bits.