Many of the common phrases for this increasingly common concept are discussed here.

TV sitcom writers undoubtedly have a name for it. They have names for literally dozens of jokes that are used, in different situations, on sitcoms again and again. When you hear the descriptions of these jokes you suddenly realize how familiar and incredibly recycled they are.

Now if I could only find that page of sitcom jokes and their names.

My friend told me a story yesterday about an embarrassment he experienced at work. He and a smartass coworker had been discussing the lack of sphincter control older men tend to have (not being an old man, I cannot say whether this is true or not). These two laughed and carried on about old man farts and whatnot for a few minutes, generally making fun of their elders.
Shortly thereafter, two customers entered the store where these two jokers work, and when my friend went over to assist them, out of nowhere, he farted, tooted really, quite loudly. He said he had no warning, and no explanation. He quietly apologized, regained his composure after a few seconds, then carried on with his work. The customers left the store laughing, and my friend was beet red with embarrassment, especially after having had the wiseass conversation about sphincter control not ten minutes prior. We call that "instant karma"