Traumatic Toggle

If you received a super power or Easy Amnesia -- or changed bodies with someone else -- due to a Freak Lab Accident, lightning bolt, blow to the head, or other traumatic event, experiencing the exact same trauma a second time will take it away, just like flipping a switch.

This is of course utter nonsense. Brain damage, for instance, does not appear on odd-numbered blows and disappear with even-numbered ones, it just gets worse. But that doesn't stop some hack writers from using a frying pan as a cast-iron light switch for amnesia or Personality Swaps.

More and more frequently, plots play on the automatic expectation that a Traumatic Toggle will always work, especially when that belief is held by the other characters in a story.

Anime and Manga

 * In the "Am I Pretty?" storyline of Ranma ½, Ranma undergoes a personality change after a blow from Akane causes him to hit his head on a rock. The rest of the cast automatically assumes a second head blow will undo the personality shift, and much of the comedy of the plot comes from repeated attempts to assault Ranma.  None succeed, and it takes another accidental head blow to undo the change.

Live-Action Film
"Johnny Pollack: Punch me one more time."
 * In the 1982 Scott Baio vehicle Zapped!, Baio's character gains Telekinesis from a lab accident; at the end of the film he suffers another similar accident, which apparently removes the powers -- or so everyone assumes.
 * Happened in Déjà vu (1989) with a hitman... who accidentally had his head bumped and forgot everything, so when he was reminded of his cover identity, he thought he really is a "professor of enthomology in transit to Sumatra, to catch butterflies". And then hit his head again. And again. Eventually, when he was caught while not knowing what he is doing there, and the mark's thug tried to beat answers out of him, John starts to remember something again: