I found the bug when posting on area51.
See this thread: http://area51.phpbb.com/phpBB/viewtopic.php?f=3&t=23102

In the 2nd post in that thread you see this: The URL for this thread is: amp;t=23102

I was trying to make part of a url bold to highlight it and it got mangled.
The code I used is below:

http://area51.phpbb.com/phpBB/viewtopic.php?sid=&f=3&t=23102

Now that I think about it its pretty hard for BBCode to parse that the way I wanted it to.