Reversion To Stereotype

A long-running sitcom character has evolved over the years and escaped the broad caricatures of his earliest incarnation. However, under stress, he'll suddenly revert to his slapstick characterization from Season 1. In some cases he'll go even broader than that.

Often invoked to short-circuit a major change in the show's status quo (because Status Quo Is God), and sometimes never referred to again.

Examples:

Live-Action TV
  • In an episode of Mash, Hawkeye Pierce and Margaret Hoolihan have a torrid one-night stand, and by the next day she's suddenly reverted to the hyper-patriotic shrew of the early episodes, scaring Hawkeye off.
  • In The Big Bang Theory, Howard Wolowitz's creepy/funny Oedipus Complex not only reappeared when he became engaged to Bernadette, it frequently roared right off the charts, where no neurotic-Jewish-man-with-mother-issues had ever gone before.

Comic Books
  • At the conclusion of the 1998 Spider-Man story 'The Gathering of Five/The Final Chapter', J. Jonah Jameson (who'd received much Character Development over the years, marrying Marla Madison, being portrayed as a man of principles and a defender of the First Amendment) reverts to being a one-note character who thinks Spider-Man is a menace at an extremely awkward moment. Similiarly, Aunt May, when she is Brought Back from the Dead, is also reverted to a caricaturish stereotype of an old lady who's afraid of Spider-Man despite all the character development she had received previously.