I'd always heard inverting a binary tree thrown around as some kind of absurdly hard problem. I took a look at it and it was trivial. I was able to do it on the first attempt with no preparation. (And the point of these interviews is that you study for them, right?)

It's a contrived scenario, but the whole point is that it measures min(a,b) where `a` is your ability to think, and `b` is your ability to prepare (and memorize answers ahead of time). (I'd personally try to find ways to measure `a` instead of `b`, maybe by asking questions people wouldn't have heard before.)