<p>Do you have a link to the question?  Bounty is taken away from the user's rep when the bounty is offered.  This could reduce the user's rep below the bounty amount. </p>

<p>See also: <a href="http://meta.stackexchange.com/questions/16065/how-does-the-bounty-system-work">How does the bounty system work?</a>.</p>