How would you even know when things like that happen people don't usually come out and say it in blunt terms like that.

Someone who scores pretty well objectively and is getting subjectively tanked by one person on the loop is something to explore more deeply.

3 out of 4, there's something legitimate there. It's possible to be coached through some of the objective parts of many interview processes, ours included. That's readily true for an internal referral (and could be innocuous rather than malicious), and even someone using glassdoor to prep could maybe skate by a couple questions, but someone will usually sniff it out subjectively.

1 out of 4 it's a "personality clash" which could be code for being threatened by someone's technical superiority, rather than simply being annoyed by someone's superior PITA-ness

If the same employee is constantly on the personality clash side of things, it's easy enough to remove them from future loops (and possibly the company, if their inferiority complex is rooted in actual inferiority).

Yep went for a job recently where I had several more years experience that the person in charge of the team :-)