Sometimes I wish interviewers were tested. E.g. mix some current well-rated colleagues into the mix, and see how they rated them. Of course that would only work in quite large companies.

But even mock interviews with current staff they know are current staff might help, as a means of weeding out questions that current, well-performing staff would fail.

I don't even blame these interviewers - most of them have never been given any training in how to interview, and it's not a skill they've ever been properly tested on in most cases. It's cruel to both sides to put untrained interviewers in that position.