Billion Laughs DoS Attack
Most browsers fix this... but here is a handy program to find ones that do not.
http://en.wikipedia.org/wiki/Billion_laughs
The server will return offensive XML by default at any request. Navigating to /lol.html serves an HTML page to see if DOMParser can choke on it.
As of this writing, the Firefox DOMParser appears to be effected. (Cheers to @davejohnson for finding that!)