Is there a penalty for misplaying?
The penalty for misplaying a card, is that now your opponents know you have that card in your hand. Besides that though, the official rules (2003) state the penalty (among other listed penalties):


  If a player plays a wrong card and it is noticed by any other player, he/she must take the card back and take two extra cards from the DRAW pile. Play continues with the next player in turn. 


It appears as if the penalty section was removed from the 2008 printing.