Here's a weird one. Have a content type with about forty fields. When I click Preview on an empty form it gives the error "An illegal choice has been detected. Please contact the site administrator." which you're sure to agree is not terribly useful. I took a look at the watchdog logs and saw that the field throwing the error is a required integer checkbox that had no items selected. So instead of saying "You must select one of the items" it says "sir you are a smeghead" - not very useful, Drupal.

With Drupal, if you have a content type with lots of records you can run into a problem where you start getting lots of "server has gone away" errors from MySQL. This stems from queries getting too long and MySQL giving up before the query has finished loading. So, to fix it, just edit your my.ini or my.cnf file to add the following line to the [mysqld] section:

• max_allowed_packet=24M

There are other settings that can make a difference too, but this seems to be a good starting point.