For me the big one is whether people in the interviews have the same mission. I went to an interview with a large investment bank where a friend turned out to be one of the interviewers. I thought highly of him, so naturally I thought his colleagues would be similar. He gave me the speil about what he does, sounded reasonable.
Then his five teammates spent an hour each with me, and they had 5 different stories about what the team does. They didn't seem to know that other teams in the same firm did the same thing and were in the news about it. Every single person gave their own version of how they fit in the team.
In the end the MD gave me a grilling about some totally irrelevant stuff, and then gave his version of the team.
I didn't proceed with the job. Caught up with my friend, and the MD is gone, along with most of the team. Friend is in another division. Another friend joined and left the group, which is yet another red flag.