Canon EOS 5D Mark III Instruction Manual (en)

Troubleshooting Guide
If you change the shutter speed or aperture during movie shooting, the 
changes in the exposure may be recorded.
Zooming the lens during movie shooting can cause changes in the 
exposure regardless of whether the lens’ maximum aperture changes 
or not. Changes in the exposure may be recorded as a result.
If you move the camera to the left or right quickly (high-speed panning) 
or shoot a moving subject, the image may look distorted.
Flickering, horizontal stripes (noise), or irregular exposures can be 
caused by fluorescent light, LED bulbs, or other light sources during 
movie shooting. Also, changes in the exposure (brightness) or color 
tone may also be recorded. In the <a> mode, a slow shutter speed 
may solve the problem.
To shoot still photos during movie shooting, using a CF card 
compatible with UDMA transfer rates is recommended.
Setting a lower image quality for still photos and shooting fewer 
continuous still photos can also resolve the problem.
Shooting still photos during movie shooting will cause a discrepancy 
between the actual time and time code. When you want to edit a movie 
using time code, it is recommended not to shoot still photos during 
movie shooting.
The exposure changes during movie shooting.
The subject looks distorted during movie shooting.
The image flickers or horizontal stripes appear during movie shooting.
When I shoot still photos during movie shooting, the movie shooting stops.
Time code is off.