How to finalize an update on internal server error?

Your server used to run fine and, on upgrade attempt, goes wild with the infamous message '500 Internal server error' or similar. What should you do?

Check file permissions

At Fineground the support team had to manually chmod updated scripts to 755 to bring the server back to life after an update.

Check the .htaccess file

This is a very common cause of internal errors. Remove every instance of such a file, or comment out every directive they contain. If internal errors disappear, then the problem was coming from there.