Continuing the discussion from The application is frozen, where are the variable files located?:
In the above topic, the user was concerned about "large Regex" searches in KM. @peternlewis identified a rare case he called "pathological regex", which I got a chuckle out of the name (but not the behavior).
So, his link had a reference in it to:
Runaway Regular Expressions: Catastrophic Backtracking -- RegexBuddy
If you are a RegEx coder (or use RegEx by others), you may want to give this article a read. It's good to know about, and has some tips on how to avoid.