I got busted the other night because I dropped my phone in the cinema, had to go back and admit I hadn't paid much attention to my allocated seat, so it was more of a job to find the phone. Still, the cinema hadn't been full and it had made more sense to me to sit in a row that suited me and avoiding sitting behind and in front of other people.

ETA (hit post too soon): I aim to miss the adverts and catch the trailers if it's not a busy night, although, as you say, for some films they don't have such a long run-up.

I can think of very few occasions where people are clearly going from screen to screen. I have occasionally walked into the wrong screen myself and then walked out, but I didn't get as far as taking a seat when the film was running, because I realised my mistake.