Just heard this one on the way to work this morning (TWIT podcast).

If you type "File:///" into any OS X application, it will (probably) crash the application. There is evidently a background service which interprets the string as a reference to an external file (File:// is the beginning of a file reference, the third / causes it to break).

The problem is exaserbated by the fact that apps like the OS X IM client will crash if somebody sends you the text. If you then restart the client, it will crash, because the text is in the chat history, which gets reloaded. You need to manually clear / delete the log, before restarting the IM client. Twitter clients are also affected, for example, as are word processors, text editor etc.

The problem also affects Safari (and other web browsers?).

Well I am reading this with Safari and it is fine. It must be something else that is crashing your system.

I read that it's a bug specifically on one version of OSX (I forget which) and affected programs that used the system spell check. It was all over G+ yesterday.

Note the capital F is important. Basically the parser isn't case sensitive, but the OS is.