In the night from 3rd to 4th January more than 2000 achievements have been deleted. This was either caused by Steam reporting less achievements (and therefore deleted ones) for whatever reason or our MySQL database had a bad night and the queries were returning nonsense. I hightly doubt it was our fault, so... I'M LOOKING AT YOU STEAM! However all attached data like calendar entries, report flags and of course all the unlocks were gone, too. I've spent two days (!) and two nights (!) to restore all the wrongly removed data based on an older backup and Google's website cache. I can tell you, that was a sisyphean task! We've now also restored more than six million unlocks (!) during the last maintenance downtime.
We're not done yet, the crawlers are currently NOT running, i.e. no updating possible. But the website is accessable again for the time being.
Another consequence is the fact, that I've implemented tracking of deleted achievements. This will help in restoring them should the need arise ever again, but also might result in a nice view of -well- deleted achievements next to the newest ones etc. Keep an eye to the comments for more updates.