Light flicker

Mains-powered lights and cheap LEDs cycle far faster than the eye notices. Your shutter speed and frame rate have to divide evenly into that cycle. When they land out of step, each frame catches a different part of it, so the picture pulses or shows bands sliding through it.

The fix is arithmetic. In 50Hz regions shoot at 25 or 50 frames a second with a shutter of 1/50 or 1/100. In 60Hz regions use 30 or 60 with 1/60 or 1/120. High frame rates for slow motion make flicker much more likely, and lights sold as flicker-free run at a rate high enough to avoid it.