Actually, not quite true – I found this topic after some digging and it resolved it for me:
https://meta.discourse.org/t/user-badges-500-internal-server-error/22459/8
Pretty much, there should be a default group for every new badge created – if not, user’s badge page will crash if he has a badge that has no group. Since there was no warning or default, I didn’t think much of it about assigning it a group.
Similar to a .NET form where you have radio buttons - always have 1 checked by default or bad things happen.