I was trying to fix an issue with the like button, and made a bad edit which killed the forum for everyone except people currently logged in using the QC-old theme. Unfortunately that meant I couldn't get back in to fix it, and had to wait for a backup to be restored.
For the technically minded, I was deleting the part of a patch file that caused the Like button not to install in the QC-old theme. I decided to comment it out rather than deleting, but used the wrong comment convention (the file is mixed XML and PHP). Unfortunately, the file affected was the one that builds the framework for every forum page...