Misreading your hand when knocking in Gin
You are not forced to "correctly" play the 6c, actually sometimes (rarely) it is an expert move to discard a card that is part of a meld. 

For example, assume you knock while holding the run {3c,4c,5c,6c} and you know that your opponent is holding the 7c and 8c.  by discarding the 6c you deny your opponent a layoff of 15 points. 

From RummyTalk, which is one of the best online Gin-Rummy strategy guides:


  sometimes it is wise to discard from a run when you are knocking. This can prevent lay-offs that could cost you dearly. There may be times when your opponent has nine melded cards and is holding up the one card that can be layed-off with your hand. In that case,
  breaking up the run will cause him to have to hold onto that card eliminating the underknock.