This may be a dupe (or sufficiently similar to) these 2: http://meta.stackexchange.com/questions/15637/bug-quoting-multiple-separate-blocks-sometimes-locks-up-the-browser-with-javascr and Firefox freezes (script execution uses 100% cpu) upon trying to quote, but those are a year old and have no answer.
I posted an answer on cooking.stackexchange.com. (I presume the error still goes to this meta: please move it if I'm wrong).
In this answer I wanted multiple blockquotes. Whenever I tried to achieve this by just working in the edit box and using the blockquote button, my browser (tested in both IE and Chrome) would freeze up when I started the second blockquote. I worked around the problem by doing the thing in stages and with a notepad window open as well, so I got out the answer anyway, but that's obviously no good.