Magic String for OSX 10.8 (mountain lion): "FILE:///"
Entering "File:///" into any text field in 10.8 will crash that app. At least one letter in file must be capitalized for this bug to work.
For maximum troll send an iMessage with "File:///" to a friend to crash their Messages app. They wont be able to relaunch the app until the message fades into the chat history (spam a dozen lines or so from iOS) or they go to Library and delete the history.
This bug will also crash the crash report. Derp.