I am sure that this thread has been done before many times, but I've not seen one recently, and I love a good, funny story.
What are the silliest things that you GM's do to mess with your group?
I, for instance, love to take a lot of scrap paper with random text and stuff on it and make a giant pile under the notes for the session, so a 6 page session may have 30 pages there in a big, intimidating stack. Then, as the PC's make choices, I'll go through and randomly remove two or three sheets at a time every time they make a big decision or when they do something random. I'll ask them before they leave a room if they want to search any more, and when they say no, they leave and BOOM, five pages gone.
It's always funny to see them get worried that their somehow messing things up, cause I never tell them what exactly is getting removed. Sometimes they figure it out and play along, but it's still fun. In the end they always find it funny and it makes for a nice joke for new players!