The worst thing you could say is, likely, nothing at all.
My interview group was about 20 students. The vibe amongst us was very open, and we all seemed to be relaxed and jovial prior to the one-on-one interviews. When all of us had completed the 2 sit-downs, we convened at the end of the hall and waited to be escorted back to our main holding area. There was a flurry of, "How did it go?", "Did you have Dr. ____?", but over the din I heard a faint sniffle. One girl was ACTUALLY crying, into her hands, slumped over on the bench. I pushed past people and asked her if she was alright (which, to her chagrin, drew attention to her slight breakdown), and she tells all of us that, "...it was AWFUL." Apparently, her interviewer--out of the gate--said, "Tell me everything you know about organ transplantation." I mean, I get it -- she didn't even warm up with a greeting, but apparently this girl said NOTHING (apart from lots of "uhhh" and "errrrr"). At LEAST just start to rattle off whatever it is you DO know....
I don't know if she started crying before she left the interviewer's sight, but her nerves were SHOT for the remainder of the day.