I've had a couple of interviews that were bad because I fluffed them, but the worst was a 4 stage process I went through a while back.

Development hub for an international org, 1st stage was a phone call with high level questions. Stage 2 were online coding tests, which I passed. Third - another phone call. Finally, a visit to the office. I was informed that I was the only one to get this far after the other filtering. This is where it all went wrong.

I'd been led to believe this would be a reasonably informal chat (around an hour or so) to fill in some of the detail of what I'd already been given. It wasn't. It turned into 2+ hours of the most intense grilling I've ever had. Felt like I'd gone 12 rounds by the end. Another coding test in the middle of it. The interviewer seemed to be enjoying the opportunity to show that he knew much more than me and seemed to be trying to catch me out, rather than really discover what I knew.

By the end of it, I didn't want the job and I didn't want to report directly to someone who seemed to thrive on making life difficult to boost his own ego.