NPC Amnesia

Your protagonist in an Adventure Game walks up to the Big Bad's base. A guard halts you at the entrance. You're presented with a Dialogue Tree; you select "I'm the new recruit." He laughs and notes you're pitifully out of shape for the job, then shoos you away.

You walk up to the same guard again. You're presented with a Dialogue Tree; you select "I'm here by order of the commander." He asks for your authorization papers. You don't have any. He chases you off.

You walk up to the same guard again. You're presented with a Dialogue Tree; you select "I'm here to fix the boiler." He admits that the boiler has been acting up lately, and lets you through, apparently forgetting that you just tried to con your way past with two other excuses.

What happened? Well, it would be cruel to give you a game over (Nonstandard or otherwise) because you selected the wrong choice, without at least a hint of the right one. It would be even crueler to make the game Unwinnable just because the guard knows you're a liar and doesn't trust you on the second visit. And it would be too simple to let the player just shoot the bastard and walk in uncontested. So they stretch your Suspension of Disbelief and give the guard NPC Amnesia. Guess The Guards Must Be Crazy.

This could be considered one of the Acceptable Breaks From Reality if there weren't at least two other options -- wait for the guard change (which could be a simple Palette Swap after you've exited the screen), or have a screwed-up conversation force you to find a non-conversational route past the guard instead. Like getting in through the window with your handy Grappling Hook Pistol, find the secret passage, etc. Or just let the player kill the guard, because that's what they'll want to do anyway. But this involved a lot of extra programming work, and the deadline is Christmas. Implementing NPC Amnesia is easier.

A subset of But Thou Must. Compare Welcome to Corneria.

Action Adventure

 * In Star Control 2, you can ask a question as often as you like, unless you offend the person you're asking when you ask the first time.

Adventure Games

 * Subverted, unsurprisingly, in Grim Fandango. Part of the Rubacava section requires you to go through a security checkpoint, and the first time you go through the guard has you place all of your items on the nearby table. Later, after you've annoyed the guard so much that she throws away an item you need, you can return to the checkpoint and attempt to go through again. The guard appears to mindlessly repeat the earlier conversation about placing your items in the table...until she makes it clear she does remember you by following those instructions with a suggestion to "jump out the damn window!"
 * Nearly any "okay, what's the password?" question will have several wrong answers, and it's not uncommon that you can't learn what the password really is until after you've given at least one wrong one (you didn't even know you had to find one until then). The guy asking never suspects you're just guessing, no matter how many bad passwords you give him.
 * Indiana Jones adventure games normally advert both the password variant and this trope in general by making you fist-fight any guards you fail to bluff your way past. However, in Indiana Jones and The Fate of Atlantis, there's a sequence where you can only learn a password by first admitting you don't know it (this will give a hint about where to find it).
 * The Longest Journey had a few of these, the most blatant being a high-strung secretary who won't let you into the Corrupt Corporate Executive's office. You can go through every option in the Dialogue Tree (all of which will fail), then leave, come back with a pizza and pretend to be a delivery girl (in the same set of clothes), and he'll let you through.
 * In the second Tex Murphy game, Tex is on the phone trying to convince another man to meet with him. This requires a very specific set of (lying through your teeth) conversation steps, and if you say the wrong thing at any point he'll hang up on you. But don't worry, you can call him back two seconds later and repeat the process from step 1; he'll be none the wiser.
 * And if you've done the whole thing right and convinced him to meet you, but forgot to use the address tracker to find out where his home is, well, you just do the whole thing again.
 * Ridiculously common in the Ace Attorney games. In this case, it's also PC Amnesia, as you can repeat an entire conversation in the exact same way multiple times. Sometimes, taking advantage of this is even necessary to continue the game.
 * Subverted in Sam and Max.. you need a password to get into the back room, and the trope works just as described even though you are obviously not a member of the mafia (as they all wear giant bear heads). However, the first thing that happens when you get through is that they're impressed you were able to steal the password, and are offered a position in the mafia provided you do some other jobs for them. Incidentally, one of Sam's initial guesses before you get the password is "Swordfish"
 * Averted in Gemini Rue. At one point, you have to ask a fellow at the front desk of an apartment building the room number of Matthius Howard. But he won't give it to you without (what he thinks is) a legitimate reason and there is in fact a correct solution. If you fail to provide one, he'll refuse to speak with you for the rest of the game. At which point, you can call your partner, Kane to call the guy and ask for the number, which he'll relay to you. Played straight later, though. In which you have to convince a smuggler to meet up with you, and no matter how many times you botch the conversation, he's willing to give you another chance to convince him despite his apprehensions.

Puzzle Games

 * Happens frequently in the DOS game God of Thunder.
 * To be specific, it happens:
 * In chapter 1, when you go to get the bridge repaired.
 * In chapter 3, when you try to enter the.

Role Playing Games

 * RPGs often have a similar, if simpler, situation where an NPC will ask a yes or no question. You can answer "Yes" and get some information, then come back and answer "No" to get different information that might also be important. Or answer "No" first. It's all the same to the NPC, who doesn't recognize you as the flip-flopper you are.
 * Yuffie appears to the party as a random battle encounter. If, during the post-battle scene, the player answers wrong, she'll disappear, stealing from you. She keeps coming back with the same dialogue and continues to steal from the party, until her entire dialogue's answers are chosen correctly and she joins the party. Although at that point, she will acknowledge the previous encounters by offering to return "some" of the money she stole earlier.
 * Averted in Breath of Fire II. When you're trying to smuggle your partner out of town in a garbage can, the guard will stop you and ask what you're carrying. Any of the three responses will work. Later on, the party is asked by a monk what the true name of their god is; answering correctly will let you skip a boss fight.
 * The latter case is less an example of this trope as there is an (incredibly obscure) way to find this out.
 * Or, you could just have
 * Averted in at least one instance in Fallout, where you can lie about having done a particular quest. If you chose to lie and get found out, you can go and do the quest, and come back only for him to tell you "I think you're just lying to me again. Go away."
 * Also one instance in Fallout 2, where if you fail to convince the guard first time and try again, the guard remarks that you didnt even walk behind the corner before coming back and giving another excuse.
 * Averted in Tales of Symphonia. At one point you have to deliver a letter to a king to help one of your party member friends, but the king is ill and the guards will not let you inside his castle. Just after the first two guards deny you entry, two other guards appear and tell the first two "Your shift's up.". This allows you to leave and come back shortly, after you get the help of a  who allows you to pretend that you're helping her deliver sacred wood inside the castle.
 * Numerous examples in the original MOTHER, a.k.a. Earth Bound Zero, often just for fun but there are some particularly complex trees which must be solved through trial and error in order to proceed.
 * A required feature in Assassin's Creed. You can kill someone,turn and corner and hide in a haystack, wait for thirty seconds and then do it again. Nobody notices the man in white when he emerges from his hiding place.
 * In Megaman Battle Network, the quiz givers will ALWAYS repeat the exact same lines, and give the exact same questions. always. Apperntly they have forgotten that you have just done their quiz and failed, odd, as you would think that they would consider a rare item as a prize a reason to remember those who have taken the quiz.

Live Action TV

 * Accidentally parodied by Saturday Night Live with its Land Shark routine.

Western Animation

 * Mocked by The Simpsons - in one episode, Homer takes a free sample offered by someone in a store, but is refused a second. So he comes back with a fake moustache on, and is refused again. Until Homer comes back on the other side of the screen, and it's actually someone else who looks like Homer with a fake moustace. You Had To Be There.
 * Also done in Family Guy.
 * And Spongebob

Real Life

 * The former head of the U.S. Secret Service tells the true tale of how a group of fake check passers went up to a clerk, got him to cash some checks, then went outside, switched their hats, went back in, cashed more checks under a different name, went back outside, switched shirts, went back in...etc.
 * This is a real quirk of human memory, especially in situations like this. People don't pay attention to things that don't directly concern them very often. A hilarious test into memory involved people signing in for a psych exam, but the real exam was memory. One researcher handed them the forms, then ducked behind the desk and a different person (including glasses, a beard, and a different shirt and hair style) collected the papers when they were done. You could switch out three times on most people, and no one would notice. In a situation where you meet lots of people (retail/secretaries) it is even easier to fool people.
 * A similar experiment was done with a "stranger asking for directions" format. The first experimenter would ask a stranger for help figuring out a map, and in the middle of this a couple people carrying a huge object would separate the pair, as the first experimenter switched out for the second experimenter, who just stepped in and continued asking as though he'd been there all along. They managed to change a white woman into a black man and no one blinked an eye.
 * There is a YouTube video of Derren Brown showing off this quirk, although, to be fair, he's a master at picking susceptible people out of a crowd . For an added bonus, the object being carried is Derren's own portrait. And yes, they switched Derren for a black man and an Asian woman and the person wouldn't blink an eye until he left.