Canon EOS-1DX Instruction Manual page 395

Playback Problems
Part of the image blinks in black.
[33: Highlight alert] is set to [Enable] (p.255).
A red box is displayed on the image.
[33: AF point disp.] is set to [Enable] (p.255).
The image cannot be erased.
If the image is protected, it cannot be erased (p.279).
A voice memo cannot be played back.
Set [85: J/K button function] to [Play memo (Hold: Record
memo)] (p.338).
The movie cannot be played back.
Movies edited with a personal computer cannot be played with the
camera.
When the movie is played back, camera operation
noise can be heard.
If you operate the camera's dials or lens during movie shooting, the
operation noise will also be recorded. Using an external microphone
(commercially available) is recommended (p.243).
The movie has still moments.
During autoexposure movie shooting, if there is a drastic change in the
exposure level, the recording will stop momentarily until the brightness
stabilizes. If this happens, shoot in the <a> mode (p.231).
Troubleshooting Guide
395