It is possible to "unbump" a thread if, after someone bumps it, you reply to it and delete your reply.
I'm not really sure if this could be abused, but it is still a minor glitch.
/edit
I just tried rebumping the thread that I accidentally unbumped, and for some reason the system thinks I have bumped a thread in the last 5 hours. I have not.
This is the thread I am referring to.
edit/
