As the film passes through the gate occasionally friction can cause small slivers of celluloid to break off and stick in the side of the opening. This debris is called hairs. A "hair in the gate" will remain in front of the film and create a dark line that sticks into the edge of the film frame as the camera is filming a shot. A hair can ruin the shot and is almost impossible to fix in post production without being painted out digitally.
Because of the intractability of this problem the 1st Assistant Camera (in the UK this position is known as focus puller) will open the camera and examine the gate for hairs at the end of each shot. Normally the assistant director will call out "check the gate" when the director is ready to move on to the next shot. The crew will wait until the focus puller calls out "gate is clean". If the gate is not clean, it will be cleaned with orangewood sticks and canned air and the crew will take the shot again. A good camera crew usually checks all cameras and magazines during equipment checkout with scratch tests using fogged film rolls - this will catch out any possible scratch or hair problems caused by faulty equipment. However, a variety of other factors including environment, humidity, type of film stock, camera position, film ridging, and lacing can each be responsible for a "hair in the gate". Generally, skilled crew and regularly inspected tested equipment make this a rare occurrence, and it is not unheard of for assistants to go for months without seeing one, even on large-scale shoots. The cinematographer Oliver Stapleton recently praised his assisting team on Casanova for not having a single hair or scratch for the whole shoot, even though large portions were shot on dusty exteriors
Video cameras do not have this problem, as any malfunction to the sensor will render the entire system useless. The new Arri D-20 system, however, does have removable lenses and a rotating optical shutter, which does mean that the CMOS sensor can be exposed in much the same way as a film gate and thus needs to be kept assiduously clean.
There are three different commonly accepted ways to check a film gate for hairs. Some may be better for certain logistical reasons than others on particular shots, and different assistants have different preferences for which to use first. It should also be noted that if a hair is in the gate but outside of the aspect ratio being filmed (if for instance, there was a tiny speck at the bottom of a full 1.37 ratio gate but it was framed for 1.85 with common center), the hair may not always be considered enough of a problem, though usually any evidence of one is enough not to risk a lost shot.