I have to disagree with Cryptocat:
Yes, there is a chance that it will be stuck for a very very long time, especially if you are sending a tiny amount of BTC.
It is possible to double-spend the Bitcoins and the first one to be accepted in a block will come through.
(the same client won't let you make a second transaction, you could import the private key to a second one or use some program to edit a local client)