Canon EOS 5D Mark III Instruction Manual page 371

The exposure changes during movie shooting.
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.
The subject looks distorted during movie shooting.
If you move the camera to the left or right quickly (high-speed panning)
or shoot a moving subject, the image may look distorted.
The image flickers or horizontal stripes appear during movie shooting.
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.
When I shoot still photos during movie shooting, the movie shooting stops.
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.
Time code is off.
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.
Troubleshooting Guide
371