I'm running popularity contest and it seems to work okay generally - apart from when creating editing or deleting posts / pages. I'm using the latest version on WP2.1 (although it did exactly the same before I upgraded).
Under any of those conditions I get the following message
Warning: Invalid argument supplied for foreach() in /~/wp-content/plugins/popularity-contest/popularity-contest.php on line 1313
You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ')' at line 3 on line: 1321
On a new post or edit, the action takes place anyway - so it's not serious, but inconvenient. When deleting a post or page it doesn't actually work.
I'm guessing there's an incompatibility with another plugin .