When you discover that they're the grandfather of a person born in the 1800s, and the profile is green privacy level, so you can't even edit it. And then you realise there's about 40 other linked profiles in the same state, all with GEDCom cruft.
And when you ask to be added to the trusted list (hoping that the profile manager who hasn't made any changes since 2014 won't respond, so you can open up all these profiles) ... They add you to the trusted list of that single profile. So you can only edit that one, and not their wife "Unknown Unknown born unknown in unknown, died unknown", and their three children all named Unknown, born and died unknown.
THAT drives me crazy.