1) Character's been bragging how he got one over on his boss. Boss calls him into the office and begins repeating characters's words verbatim, often with the character shrinking at every sentence. Oops.
2) Character thinks his wife/kids/friends doing something wrong behind his back. He jumps up with a Gotcha! just as they are going to present him with something, food/tickets/car/party he always wanted. It's usually ruined, and character shrivels in response. Oops.
3) Character is trying to calm a crying/fussing child, begins talking and acting like a baby. He soon gets into the role, forgetting the child is there, is spotted by friends/public, tries in vain to hide by shrinking. Oops.
Often seen on The Flintstones and Donald Duck.