Here are a couple of problems that I've come across in a couple of the themes we are using, Blix and Tarski - both relate to commenting.
In Blix, when a comment is made on a post and submitted, it just leads to a blank page. If you press back and submit it again, it says there is an error and this comment has already been submitted. If you then refresh the post, it shows that the comment was actually sucessful...although it is working it would seem, it's not particularly ideal!
Secondly, inTarski, the comments on a post were actually being displayed on a different post. We manaaged to attribute this to the Recent Posts plugin that the user was using and it works when that is switched off. Again though, it's not ideal as the user was really quite like to display recent posts in his sidebar!