See what happens if you expand the comments on this question, we get this prime example of a math overflow:
Of course, this is due to a \$ problem, but still, it seems to be a flaw in the CSS that this is possible.
Note that the overflowing math does get cut off, eventually, at the edge of the sidebar. Shouldn't the
overflow property be set up such that it gets cut off before that happens? The whole comment thread gets reflowed if there is one broken comment.
The same seems to apply to answers, math is allowed to overflow into the far edge of the sidebar.
This happens on Chrome Stable and FF 25.
Remark (added on Oct 4, 2015): This bug report appears to be still pending.