One that hit me hard was a homework assignment that I absolutely aced. It was timed and I was super proud of myself for not only getting all of it completed including the bonus questions with what I considered very good quality code.
But I hadn't talked to anyone in the company prior to them giving me the homework. So they are impressed and move me on to the next stage, I talk with them and immediately find out that 60+ hour weeks are the normal for them several months out of the year. So my time on the coding exercise was wasted, a simple phone screening where they gave me details of the position would've saved me from that and I now insist on a quick chat with the hiring manager to "find out if I'm right for the role" prior to completing a non-trivial exercise.
But I hadn't talked to anyone in the company prior to them giving me the homework. So they are impressed and move me on to the next stage, I talk with them and immediately find out that 60+ hour weeks are the normal for them several months out of the year. So my time on the coding exercise was wasted, a simple phone screening where they gave me details of the position would've saved me from that and I now insist on a quick chat with the hiring manager to "find out if I'm right for the role" prior to completing a non-trivial exercise.