After running my forum for a couple of months, I decided to set up emailing, so that users could get their passwords emailed to them if they forgot. To test that it was working, I logged out as administrator, and requested my admin password be sent to me. This caused an error on line 181 of the password emailing include. I didn't take a note of the exact error, because I wasn't all that concerned at the time. It was only when I tried to log back in as administrator that I discovered the problem: it had corrupted my password!
I knew it wasn't because I had forgotten it, because I had a backup of the database running on a different server from only a few days ago, and I could log on to that with the password I knew it to be. Using the same password on the corrupted forum, resulted in failure. Thankfully I fixed the problem, by downloading the database and copying a known password (and salt) from a different member whose password I had reset 30 seconds before I had logged out and become locked out.
Having something like that happen is really bad.