It seems to be standard for Drupal, at least in version 6, that Watchdog is enabled and used to write errors/warnings/notices to the watchdog table in the database. Depending on the verbosity of your PHP output, amount of 404s, or other information that would be going into this log, it could mean a lot of writes on your database. While you should trace errors/warnings and solve them, sometimes you may want this increased information for debugging. If your site receives a lot of traffic, this could mean decreased database performance. A solution to this is to instead utilize the core Syslog module instead of Watchdog. Here's how I got it set up in Drupal 6 on CentOS 5 servers.

This may be old hat to some of you, but it was a near-revelation to me (and tough to find through Googling) so I thought I'd share it and potentially save others some hair pulling.

If your Drupal 5 site goes crazy when you turn on CSS aggregation, try examining your stylesheet(s) for absolutely-addressed url()s. There was (and is) a bug in Drupal 5's CSS aggregator code — since aggregated CSS has to live in a different directory from the original stylesheets, all those URLs have to be rewritten. The function that rewrites them expects them all to be relative, though. So:

background-image: url(/sites/all/themes/zen/xxx/images/something.gif);

gets turned into:

background-image: url(/sites/all/themes/zen/xxx//sites/all/themes/zen/xxx/images/something.gif);

That's obviously no good. Instead, simply make sure that your URLs are all relatively-addressed:

background-image: url(./images/something.gif);

That should be rewritten properly by the aggregator.

The more complete solution, of course, is to fix drupal_build_css_cache(). There is a patch available, but since the issue has been more thoroughly addressed in Drupal 6 and 7, not much attention is being paid to fixing it in 5. And that's probably okay: in most cases it's going to be quicker and easier to simply edit your stylesheet to use relative paths. It's best to avoid patching core when possible, and this is no exception.