Immediately after the director yells "Cut!" on a film set, someone else yells, "Check the gates!" The "gates" are – or used to be, before most films went digital – the part of the camera where the film slides in front of the shutter. When you check the gates, what you’re looking for is something wrong, like a single hair or a speck of dust, that's going to ruin the shot when the film is processed...
Check the Gates