vBulletin's WYSIWYG editor does this. They have tried to "fix" it with some workarounds in the parser, but these workarounds tend to break our headline edit links (and have been giving me a headache all day). Sometimes they help the nesting problem, sometimes they make it worse. I know vBulletin has been trying to come up with a new editor recently, so hopefully this nesting issue becomes less common.