This is just a guess:
The code calls a .gif that is hosted elsewhere. Host site goes down or removes .gif, then bye casty. Same goes for the other 'hidden' emoticons and memes. Kige has been missing too. :sadnana"
That's a mistake on TD's part. There are a lot of emoticons hosted on TigerDroppings. Casty should be one of them.
If they're going to rehost Casty on another site, then I recommend Imgur. It's the best hosting site out there. ImageShack and Photobucket have been really spotty as of late with pics disappearing on ImageShack with no explanation and PhotoBucket's loser as frick bandwidth limit.