Can someone explain last year's reddit exploit to me?

Last year a user managed to inject arbitrary javas开发者_如何学编程cript into reddit's markdown syntax. Can someone explain how this was done and how I can test whether my site is similarly vulnerable?

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Blog entry on the exploit:

http://blog.reddit.com/2009/09/we-had-some-bugs-and-it-hurt-us.html

The patch that fixed it:

https://github.com/reddit/reddit/commit/1f1f0606f5b6bf14a0db55a28cfd03e1e42e3550