I've been having a very odd error on every page viewed by every user. I finally tracked down the problem.

I was getting this message.
user error: 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 1 query: SELECT * FROM aggregator_item WHERE fid = 0 ORDER BY timestamp DESC, iid DESC LIMIT 0, in /path/to/my/web/root/includes/database.mysql.inc on line 125.

It turns out that there was block trying to use a syndication feed that I was no longer aggregating. Yes, this is unusual but I thought I'd post in case anyone else had this problem.