In Killer Bunnies when a Bunny has a "feed your bunny" card on it you have until the end of your turn to feed it. If you draw a Terrible Misfortune card (plays immediately) can you play it on the bunny that needs to be fed (assuming you can't feed it)?
Yes, you may since your turn isn't over until you replace your Bottom Run card in Step (4), which occurs after you draw back up to five cards. Under the rules for Feed Your Bunny it says:
When using a Feed The Bunny card, a player places it on any opponent’s bunny in The Bunny Circle. The opponent will need the Cabbage Units and Water Units to Feed The Bunny by the end of his next turn. [...] If the bunny is not fed by the end of the opponent’s turn, then it dies and is removed from The Bunny Circle (discarded).
and for Play Immediately cards:
If a player draws a PLAY IMMEDIATELY (Terrible Misfortune) card during play, then he must stop the game, announce that he has the card, and kill one of his own bunnies in The Bunny Circle.
So, when you play a Feed Your Bunny card on an opponents bunny, they only have until the end of their turn to feed and save it. So, when is the end of your turn? The Blue Starter Deck rules breaks down the steps of your turn as:
(1) Flip over the TOP RUN card.
(2) Slide the BOTTOM RUN card up to the TOP RUN card position.
(3) Draw a replacement card from the Draw Pile.
(4) Replace the BOTTOM RUN card with a card from your hand.
(Note: the yellow/blue bunny bits rules were very confusing since they didn't breakdown the turn into steps, "A player’s turn is over when he has flipped his TOP RUN card (or played a SPECIAL card directly from his five-card hand), and has replaced the card so that he once again has five cards in his hand and two cards down on the table.")