For most WordPress upgrades, I just do an svn switch and then maybe upgrade the database and that’s about it.
Upgrading to 3.0.1, for some reason, was a bit of a nightmare. For some reason, svn thought tons of files were modified and it only did a partial svn switch (I would think that this operation should be atomic…?). I ended up with a mix of files from 3.0.1 and 2.9 and this caused me to get this error. So I ended up reverting or moving a bunch of existing files out of the way and then doing svn update and then doing svn switch again and then repeating this processs several times. I also had to replace my wp-settings.php file with a freshly downloaded copy. Then I got this error. I fixed that by doing more of the svn revert, svn switch, etc. dance. Finally the admin screen loaded and I was almost ready to celebrate, except then I noticed that my blog itself was returning a blank page and there was no error in the Apache error log (the server was returning a 200 OK just with no HTML content). After a bit of head scratching, I theorized that WordPress was trying to use an old theme and I had pretty much moved my whole wp-content directory out of the way so now I only had the default themes. So I copied my old theme files back into wp-content/themes and now it seems to be working, though I wonder how many other little things are broken.
Probably one of these days I should clean out my whole web directory and put in a clean WordPress installation. I think I’ve accumulated too many little hacks to work around bugs in old versions of WordPress and hopefully they’re not even needed anymore.
That was kind of a nightmare.