No Sneak Attacks

Wherever You Go, There They Are- Wherever the characters go, the villains can always find them. Chances are they're asking the guy in the street. But don't worry -- despite being able to find the characters with ease anytime they want to, the bad guys never get rid of them by simply blowing up the tent or hotel they're spending the night in. (Just think of it: the screen dims, the peaceful going-to-sleep-now music plays, then BOOM! Game Over!)

—The Grand List Of Console Role-Playing Cliches

Related to Mook Chivalry, except it applies to all villains. The villains will never go for a sneak attack that is really any good. Some things include:

Killing the hero when he's asleep is dishonorable—A villain would never have the common sense to simply sneak (or teleport) up to the hero while he's asleep. Common sense would tell you that a villain could easily win by waiting until the hero is asleep and then killing him.

I must never attack from behind without some sort of warning sound—It never occurs to villains to sneak up behind the hero and blast him into oblivion on what is supposed to be a normal day when the hero is off guard. If the villain attempts to attack from behind, he will always make some sort of noise, like a yell, to warn the hero to dodge or be pushed out of the way.

Compare Why Don't Ya Just Shoot Him and Final Boss Preview.

Examples of No Sneak Attacks include:

Anime and Manga

Ah! My Goddess: Mara, the demonness and arch rival to the goddesses, has demonstrated an ability to seal away Belldandy and/or her powers. It never occurs to Mara to sneak in at night or sneak up from behind and do this.
Happens twice in Code Geass: both when Nina tries (repeatedly) to kill Zero (yelling "Euphemia's revenge!!" incessantly), and when some Britannian soldier tries to assassinate Suzaku, the latter of which would have worked if it weren't for Suzaku's "Live on!"
- Also, Suzaku's attempt to kill Charles, which involves charging in like an idiot and making a long speech about crimes and despair and whatnot. There's some sadly unexplored hilarity in the fact that Charles was invulnerable by that point.

Fan Works

Actions such as stealing Yugi's puzzle while he's asleep are regularly suggested by the Only Sane Man on the Evil Council on Yu-Gi-Oh the Abridged Series. And then shot down by Marek who can't abide by any plan that doesn't involve playing a children's cardgame first.

Film

In The Cabinet of Dr. Caligari, violating this trope is Cesare's job. And if you think it's unfair that he kills you in your sleep, don't worry: he kills you in his sleep, too.

Literature

The Assassin's Guild in Discworld has explicitly defined standards as to how one is permitted to inhume a man. These standards permit Commander Vimes to set up defenses around his home and office to ward them off. He gets so good at humiliating would-be assassins thanks to this trope that they eventually stop accepting contracts for him, and use the course as a test. No, not killing him, just getting to where one can draw a bead on him is considered exceptional.
Averted in The Lord of the Rings, when Sauron does send him Ring Wraiths to attack the heroes while they sleep in the town of Bree.
In A Song of Ice and Fire there's an order of assassins who always apologize to their victims before killing them. This is a whispered "Sorry," an instant before death, but it was enough warning for one character to avoid it.

Live Action TV

Justified in Power Rangers: Rita could only get rid of the Rangers if she killed them while Morphed; otherwise their powers could just be given to another person.
In Charmed, demons have the ability to teleport in and try to kill the Charmed Ones, seemingly at will. While they do take advantage of this, they always do it when they're awake, usually in the same room, and not occupied with anything else. The idea of doing it while they're asleep doesn't seem to occur.

Tabletop Games

Encouraged as protocol for the Keeper in the Call of Cthulhu core rulebook. As the Investigators tend to be mere Puny Earthlings, devouring them in their sleep or when they otherwise can't fight back tends to be boring and anticlimactic. Individual pre-written adventure modules may play this straight or avert it, however.
Subverted in the Forgotten Realms setting, where the assassin Entreri often does kill people by sneak attacks, and on one occasion when he does cry out when attacking someone from behind, it's because he intends for his opponent to kill him.

Video Games

In one of the expansion packs to The Elder Scrolls III: Morrowind your character will be targeted by a secret group of assassins who only attack you once you've bedded down for the night. Fortunately, they have the courtesy to yell at you and wake you up before they attack.
- You get some sweet armor from the deal, too.
- Considering these assassins are supposed to be among the best (and most amoral) killers in the region, their making a noise when attempting to kill you - every single time - seems quite odd.
Subverted in Call of Cthulhu: Dark Corners of the Earth, where if the player uses the action button on the bed to sleep before bolting all the doors, they don't get to wake up - they merely get a Game Over.
- Unbolting and/or breaking down a door tends to make considerably more noise than simply opening it, of course - what's slightly odd is that the player character wakes up before they interact with the door.
In Arcanum: Of Steamworks and Magick Obscura assassins will not only stand and announce them selfs, but converse with the player before attacking. Lampshaded after you learn more about the assassins, their history and their "code of conduct".
- Likewise, several people who hang around in the bars may randomly turn out to be assassins - but it will only be determined if you choose to talk to them.
In the second Neverwinter Nights expansion, a reversal of this occurs if you agree to aid a group of formians (intelligent centaur-like ants) in wiping out a drow base. The game will take control of your character and have a cutscene where you just stroll in through the door and call attention to yourself. Not very wise if you're playing a less-than-direct-assault character.
Fallout 2 is a subversion in that you can involve yourself in the affairs of the bad guys in quite a few ways before the final confrontation, including prank calling them, giving them a chance to build some kind of profile about you. The only person who is assured to care about you in the end is the last boss.
Subverted in Super Paper Mario. The Dragon teleports in and offs the whole party during an exposition session halfway through the game. It turns out to be a Batman Gambit on his part to help the heroes gather all the Plot Coupons.
Final Fantasy:
- In Final Fantasy IV, Cecil and Rydia are hunted down by Baron - Rydia for being a Summoner of Mist, and Cecil for the treason of protecting her. The squad ambushes the pair as they rest in the inn, but wake Cecil before attacking. Cecil defends Rydia alone against them, which convinces her that his regret and compassion are sincere. This actually happens an enormous number of times in Final Fantasy IV, so often that it may actually be a major theme of the story.
- Also subverted in Final Fantasy VII when Sephiroth kills Aeris from a sneak-attack, with no warning.
Played with in Mass Effect 2. In the opening cinematic, the Normandy SR-1 detects the Collector ship, but does so too late. Played straight again with attempts to destroy Shepard's body to prevent the Lazzarrus project and again when the Reaper IFF beams the SR-2's position, leading to another Collector suprise attack. Played straight in the Arrival DLC.
The second version has appeared in relation—oddly enough—to the Spy from Team Fortress 2. In particular, the Sniper in "Meet the Spy" hears him approaching and moves to retaliate (not that it stops him from being bested and stabbed in the back anyway). Additionally, the decloaking sound was recently made much louder, which means that particularly aware foes can hear him before he strikes and fight back.
Mooks in Resident Evil 4 will actually say "Behind you, you idiot!" as they are about to attack. Admittedly they say it in Spanish, but still.
Averted hard in Sonic the Hedgehog 2006: Mephiles's original plan was to trick Silver into killing Sonic. When that didn't work, Mephiles simply teleported behind him and stabbed him through the chest with a giant energy lance.
For the most part, the Mega Man Battle Network series follows this trope, until it is subverted in the fifth and sixth games when some of the enemies finally get smarter and begin targeting Lan and Megaman (mostly Megaman) when their guard is down.
The Sleep spell in Warcraft3 puts an enemy to sleep, but attacking it ill ake it up. Its main use is to interrupt chanelling spells or remove a powerful unit from action for a while.
In Assassin's Creed: Revelations, enemies finally appear who use the same tactics as Ezio, with one exception. When they appear out of nowhere, they yell something like, "Die, Assassin!" before stabbing him. This naturally gives the player a chance to dodge the sneak attack.

Webcomics

Drowtales: Syphille falls hard into this trope, to the point where she could get the picture.
From 8-Bit Theater, two ninjas discuss the merits of shouting "Surprise Attack!"

Ninja 1: Hey, they're dead, aren't they? So what's it matter, hm?
Ninja 2: Well, there used to be six of us, now there isn't.

Played with in The Order of the Stick. Rogues yell out "Sneak Attack!" whenever they perform a sneak attack. The Thieves Guild berates their fighter for yelling out "Power Attack!" when he uses that feat.