The recruiter told me to prep algorithms, data structures, space/time complexity, concurrency - all in Java. I do Swift. I emailed to ask if it'd actually be in Java, never got a reply. Spent days preparing for something completely different, then walked in and it was nothing like what I'd been told. The exercise was actually fine and kind of interesting, but I was thrown off from the start. Everyone was friendly enough - the recruiter just clearly had no clue what the interview was going to involve.