A nasty OS X Mountain Lion bug has been discovered. Type the following followed by a space in virtually any OS X app and it will crash instantly:
The upper case F at the start is important.
The Next Web has more info. The bug is caused by OS X’s autodetection of URLs. Turn that off as described in the The Next Web article and you should be OK.
This isn’t a critical bug. The biggest risk will be somebody trolling unaware OS X users in forums or chat messages, attempting to make them type it. So now you know — forewarned is forearmed!
iPads and iPhones seem unaffected, by the way. In fact, this posting was written using an iPad. Older releases of OS X also aren’t affected.
If you want to try out the bug be sure to save files first, and reboot afterwards in case a memory leak is brought about.
Update: This also happens if you copy and paste the phrase. The bug doesn’t appear to be fixed in the soon-to-be-released 10.8.3 update for OS X.