Nowhere nearly as overused as most of the other plot devices we've mentioned in this thread, but here's a plot that has seen it's share of overuse as well over the decades: we learn one or more of the characters has a rather embarrassing middle name, and in most of those cases, if the character is male, the middle name is usually regarded as a feminine name: both Double D from Ed, Edd n Eddy and Frank Burns from M*A*S*H have Marion as their middle names.
And overused even less is if a character goes by a nickname or their last name, or even a Barkeep name, after a while, we FINALLY learn the character's real name, and it turns out to be just as embarrassing as the middle name plot device (ala Kramer from Seinfeld's first name being Cosmo, Curly from Hey Arnold!'s real name being Thaddeus, or Binky from Arthur's real name being Shelly).
EDIT: Then there's also the nickname episode, where a character receives a nickname they dislike, but doesn't have the courage to just come out and say, "Dude, dis nick ain't kewl man!"