Director Alfonso Cuorón told a rapt Comic-Con audience yesterday how one of the most memorable scenes in his classic movie Children of Men was actually a mistake.

In the scene, which you can see above, our group of subversives are attacked while in a car. The tricky part for the director was that it's all done in a single take, and a single shot. That heightens the tension for the viewer, because there are no cuts — you can tell that the whole thing was filmed in one, long sequence. But this is a nightmare for the director, because there are so many elements — the burning car, the shooting, the breakneck escape — and they only had a few chances to get it right. The take you see in the movie was their last chance, their last take — and Cuorón actually yelled CUT right in the middle of it, when a splash of blood hit the camera lens. Luckily, nobody heard him.

During a panel yesterday, he said:

We had 12 days to do the car attack scene. 10 days into it, we they were still staging it. After 12 days, we were going to lose the location. Day 11 came and there were accidents, and we could only do 2 takes a day. On the last day, we knew we were losing location next day. In the morning it was great, but an operator fell down so we only had only one more shot. We were shooting the last take, everything goes great, but then by accident the blood spills onto the lens. I yelled cut, but there was an explosion and nobody heard me so they kept shooting. Then, later, I realized that the blood splash was the miracle [in that scene].

Cuorón said that his latest film, Gravity, was full of crazy accidents like this too. "The whole film was a mistake," he joked. But you could hear the pride in his voice, behind the humble words.