Canon EOS-1DX Instruction Manual page 392

Troubleshooting Guide
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 smaller image size for the still photos and shooting fewer
continuous shots may resolve the problem.
The 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.
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