Phantom Paper Clip
High risk may mean high hilarity.
This prank will potentially screw up the printing/copying of your coworkers' documents so do this prank at your own risk and preferably when you know no important documents are going to be printed any time soon.
Step 1: Stick a paper clip into a copier and make some copies. This should give you blank pages except for a paper clip mark somewhere on them.
Step 2: Place the pages into the copier so that the machine will use the marked pages for the next copies.
Result: The next copies made using the machine will have paper clip marks on them. The person making the copies will think a paper clip is stuck somewhere in the printer/copier and will attempt to find it. After not finding anything, they will attempt to make more copies only to find the same problem.
You don't have to use a paper clip for this prank. Tape, torn pieces of paper, pencils, and other office items all work well.