I used the auto-upgrade in wp-admin to upgrade to the latest version of WordPress on one of my WordPress-hosted blogs.
It downloaded, and then presented me with a 500 Internal Server Error.
Tediously, I don’t have easy/immediate access to my error logs, so I was working blind. The WordPress site wasn’t much use. After much fiddling about and (unnecessarily) manually reinstalling, I realised the problem (thanks to Unison, my file sync tool): all the files had their eXecute permissions removed! A quick chmod -R a+x later and everything was back up again! Whew!