I seem to frequently see this happening when browsing the SO featured questions.
An answer is high-voted and even accepted, and yet the bounty is still around. In almost all cases it would seem that the OP forgot to grant his bounty.
When accepting an answer for which he assigned a bounty, the OP should at least get some kind of ajax nag. As in "Don't forget to grant your bounty". Or the bounty should be granted automatically if it was contributed by the OP.