Gin Rummy, incorrect knocking
Here's one answer from http://ginrummytournaments.com/pdfs/Rules_2012.pdf. Tl;dr if you could have knocked then you can accept his knock and undercut; otherwise game continues, he plays an open hand, and he must knock at first opportunity.


  IMPROPER KNOCK: If a player makes an IMPROPER KNOCK, with his count being
  higher than the knock card, his hand must then be PLAYED OPEN, ALL TEN
  CARDS FACE UP. He MUST KNOCK when his point count is at the value of the
  knock or below. HE CANNOT BE FORCED TO KNOCK BY HIS OPPONENT. When a
  player makes an illegal knock with more than the knock card count, the opponent
  may accept the knock as proper, provided his point count is the same as the knock
  card or lower.(example; the knocking point is 8 and a player knocks with 10 points,
  the opponent with 8 points or less may accept the knock and get credit for the
  undercut). The opponent may also lay off points to get under the knock. When a
  player announces he is ginning or knocking, or calls a number and starts to lay his
  meld down, he must continue to do so. If his hand is not a gin or his knock is not
  legal he must play an open hand. The opponent may ask that the cards be spread
  for proper viewing, but may not touch the cards himself.


For casual play however I would tend to let my opponent pick up his cards and play normally - he's the only one who's been hurt by showing them to me.