Yeah, it's not clear what he did, all he references is a twitter posting saying he lost data: http://twitter.com/garybernhardt/status/9368728335 , not how.

I think it's fair to surmise that he must have been messing with commands he didn't fully understand.

The bottom-line is that in either git or hg you have to step past the screaming sirens and big red warning signs to truly lose data.