1. Correcting the interviewer. In one of my interviews, an MD/PhD was explaining to me his experimental design, which involved training a cohort of people to do a task, then afterward subjecting that cohort to electrical brain stimulation while performing the task in order to compare results on the task. Concerned, I mentioned that the experiment wasn't controlled for people merely becoming better at the task as they had more exposure to it. The person in charge didn't approve of this, and perceived it as an attack on their experiment. Clearly, this was the point at which I "lost" the interview. Lesson: the interview isn't the place to tear apart problematic projects.
2. Being honest with the interviewer. In another of my interviews, I was discussing various volunteering projects that I had undertaken. The woman who was interviewing me was old enough to be my mother, and seemed to be approving of my extensive list of volunteering opportunities, which she had inquired about. This lulled me into a false sense of security, leading me to disclose one of my volunteering projects which involved video-game-based mentorship of disadvantaged youth. This immediately raised her hackles, prompting her to ask if I "played video games a lot". I said yes, being young and stupid, assuming it'd be absurd to hold something that I did in my private time against me. I "lost" the interview" with that admission, even though I'd been golden up to that point. Lesson: the interview isn't a place for anything other than pushing the interviewer's buttons until you get exactly what you want.
3. Asking questions for which there is no feel good answer. In yet another interview, I started to get a whiff of desperation from the interviewers regarding workflow from manager to employee. I probed the issue, then asked directly, "How is success measured here?" suspecting that I'd get an answer like "completed projects per week" or something else which would come off as overly controlling to employees. I was told there was a quota based system. This turned me off immensely, and turned the interviewer off as well for having to admit such an embarrassing thing. I still got a job offer after this mistake, but I ended up rejecting it. Lesson: don't embarrass the organization you're interviewing for by making them admit weaknesses.
4. Permitting bad behavior from the interviewer. In three (3) separate interviews for different organizations, the interviewer arrived 10-15 minutes late with no apology. In the first of these interviews, the interviewer's beeper went off, causing a flurry of cussing and an interruption of the interview for an extended phone call. This happened again later in the interview. I "lost" the interview because of the constant interruptions by the interviewer's other commitments, and didn't get the job-- maybe a bullet dodged here. In the second of the three late interviews, the interviewer notified me beforehand he'd have to stop in the middle in order to check his son's scores, as his son was in some sort of sports competition at that moment. Sure enough, the interview was in a state of anti-flow due to his constant pausing and checking his son's scores, at times cheering quietly. I "lost" this interview because of blatant disrespect-- maybe another bullet dodged. The final late interview was spared any interruptions, but was instead occupied by a bizarre egotistical rant based on the interviewer's past accomplishments, detailing how the interviewer had presented his research to a crowd of many people and made a name for himself. This was irritating, but tolerable. Following his ego-trip, the interviewer then went on to compare me and the other (not present) candidates to his son, saying how his son was the ideal candidate and that everyone in my generation other than his son was extremely entitled and should really lower their expectations because of how hard he had it when he was coming up. I can't make this shit up. I lost the interview when I failed to behave like his son. Lesson: don't be afraid to stand up and walk out of an interview if the interviewer isn't prepared to take you seriously.