One plugin that's known to cause this issue is an old version of the Yet Another Related Post Plugin or YARPP. They've released an updated version that fixes this issue which you can download from the plugin site.
Another plugin that has caused problems when used with the AddThis plugin is the Minify plugin, which can cause a stray <em> tag to show up on your page, making the whole thing in italics.
As we find more conflicting plugins we'll add them to this list.