A picture is worth a thousand words; here is what I am talking about, from the comments on this post.
By posting a very long formula into MathJax, not only have I caused my own comment to run off the edge of the screen (which perhaps I deserved), but the same has happened to all the other comments in the thread.
Firefox 38.2.0, OS X 10.8.5. Bug also occurs in Safari 6.2.7.
UPDATE I just deleted the comment, since it was inconveniencing others in the thread. Hopefully, this has left enough data to reproduce the bug.
Remark (added on Oct 4, 2015): This bug report appears to be still pending.