Created a conditional activity 'Label' and set it to only be visible to users who had put a particular string into their Interests field in their User Profile.

There were no users who had that string in the field.

This created a nasty loop where I could not refresh or move onto another page without the error message popping up.

Unfortunately I was not smart enough to grab a screen shot before I reverted to a nice fresh install of Moodle.

I'm guessing for those who have a backup, if this happens a back up to something prior to creating this logic loop would work.

Better would be to have Moodle not enact the condition until the condition was met.