A bug has been discovered with OS X 10.8-10.8.2, where some applications will crash or hang if you type File:/// (the word File capitalized, followed immediately by a colon and three forward slashes). Copying/pasting File:///, or opening text files containing File:/// can also cause the problem. This affects some third-party apps and many Apple apps including Safari, TextEdit, and Mail.

This problem actually occurs if any of the letters are capitalized (File:///, filE:///, fiLe:///, FILE:///). It does not occur with OS X 10.7.5 or older. It does NOT occur if typed in all lower case (file:///).

Although this is a very obscure bug, one that Apple will likely address with the forthcoming OS X 10.8.3 update, it has been getting a lot of press lately due to pranksters. One example is people posting links to Facebook titled something like “Mac users, check this out!” that point to File:///. Safari will crash for Mountain Lion users who click on links like that. Similar pranks involve sending Mountain Lion users an instant message including the text File:///, which immediately crashes Messages if that’s the app they uses for IM.

The current workaround for Mountain Lion users, who wish to not have apps crashed by this bug, is to disabled “Correct spelling automatically” and “Use symbol and text substitution” in the Language & Text system preference pane.

Benign as it may sound, playing around with this bug (i.e. deliberately crashing a Mountain Lion app by typing in File:///) may actually cause residual problems. It has been reported that once an app is crashed by this bug, it may continue crashing when re-launched depending on how the Mac is setup. Fixes for this issue have been posted here:
http://reviews.cnet.com/8301-13727_7-57567684-263/reverting-file-url-bug-mishaps-in-mountain-lion/