I was playing a PvP and I cast Whimsy on a Heimerdinger on the enemy team just as I died.
I didn't realize until a couple minutes later that Heimerdinger was stuck in squirrel form for the rest of the game, but could still cast spells and attack normally as far as I could tell.
Sounds like the problem was only on your end; your client never changed his character's texture/model. This is still a bug though.
© 2013 Riot Games, Inc. All rights reserved. Riot Games, League of Legends and PvP.net are trademarks, services marks, or registered trademarks of Riot Games, Inc.