The Next Web has uncovered a bug in OS X that will crash the current application almost every time if invoked. Reportedly, in applications that support text entry fields, if you type the text "File:" followed by three forward slashes in the field to indicate a file address, then the program being used will freeze and then quit.
The bug is a problem in a core system service called Data Detectors that is responsible for handling dates and locations, part of which is a checking routine for an entered URL address. When you type information into programs that support Apple's central text-handling services in OS X, the system apparently actively checks text strings with a service called Data Detectors, and part of this routine is to validate URL addresses; however, when handling filesystem addresses it runs into an error it cannot recover from, which results in the crash.
This happens when you copy and paste text as well, so if you were to transfer text that contains the offending string of characters from one program to another (even by dragging and dropping), then programs you inserted it into would likely crash.
Luckily, this should be an uncommon occurrence since it does have some requirements. For one, the bug is only triggered by a standalone instance of the offending text string, so if you type those characters surrounded by parentheses or brackets or otherwise make the first character in the string not be an "f" then there should be no problem. The one exception to this appears to be the use of quotation marks.
Also, the bug is somewhat case-sensitive. The accepted form of the text by the Data Detectors service is for the word "file" to be lowercase, so the error will only be triggered if you include any combination of capital letters.
Additionally, Apple's work flow preservation features in OS X should help manage the issue for any who encounter it. While the current application will crash, Mountain Lion's Resume feature in OS X should return the window locations and contents to where they were when the program halted. The system's Auto Save feature should also preserve most if not all of the changes made to the documents before the crash.
While most programs tap into Apple's text-handling services and therefore are affected, not all interface with these services. Older programs and some technical programs such as WaveMetrics' Igor Pro computational platform and BareBones' TextWrangler are not affected. Additionally, while the OS X Terminal utility itself is affected, the Bash shell and programs that run in it are not. Therefore, if you go to the Terminal's preferences and type this faulty string in a text field (for example, setting the default opening command) then the program will crash, but you can type it all day at the command line or in a text editor like Vi or Pico and be perfectly fine.
This issue only appears to affect versions of OS X Mountain Lion so far. In its testing The Next Web found it does not happen in installations of Snow Leopard or Lion, which I have confirmed with direct installations of both and in virtualized installations of OS X Lion.
Unfortunately, being a text-handling bug, if it does get triggered then the issue may have a feed-forward effect in the system's crash reporter and logging services. Generally if a program crashes then the system will open the crash reporter utility so you can send details of the problem to Apple. It appears the offending text string is included in the crash report and is similarly handled when the reporter opens, resulting in it crashing as well. The same may happen if you use the OS X Console utility to review the crash logs. While the ongoing system log in the console will show the crash instances and the text string, if you select a crash log that contains it then the Console will invoke this bug and likewise crash.
It is very likely that Apple will address this bug in an upcoming OS update, but until then, if you are running Mountain Lion be sure to avoid typing this specific sequence of characters.