This happened to me at a recent Google interview. It was the last interview of 5 and the guy gave me a gnarly graph problem. I was trying to wrap my head around it, but he kept interrupting me and giving me unwanted help. I ended up getting writer's block and just mentally shutdown because he wouldn't let me have time to think. He ended the interview early because of poor performance.

It made me feel like I didn't know anything and was very embarassing. It also changed my outlook on Google, and I'll never interview with them again.