Given the complexity of modern operating systems, bugs are to be expected. Some are obvious, but inconsequential, while others are catastrophic, though rare. A recently found bug with OS X Mountain Lion falls into the latter category, causing an app that supports text input to crash, simply by typing in a few letters.
The letters in questions? Here you go:
You can trigger the crash yourself by opening up TextEdit and whacking in the fateful phrase, or almost any app that supports inputting text. This includes parts of the operating system, such as Spotlight, which will act bizarrely if faced with the text.
You can read some more details in this bug report over on Open Radar. It theorises the bug is due to the spell checker, but going through the comments (and looking at the crash report itself), it may be related to data detectors. If you were hoping disabling the spell checker would help, it doesn't, at least going by other users posting on the report.
The bug appears to only affect OS X Mountain Lion. I tried to replicate the bug on a MacBook Air running OS X 10.7.5 (Lion) and was greeted by soothing stability in both TextEdit and Word.
If you're running any recent flavour of OS X, give it a go and let us know. Just make sure you're not doing anything work critical at the time...