Filming & Production
Variable frame rate
Also called: VFR
Phones, screen recorders and some cheap cameras add or drop frames as light and processing load change, so a file averages out to 30fps while never actually holding it. Editing software assumes a constant rate, lines the audio up against the frames it expects, and ten minutes in the voice is sitting behind the mouth.
The fix is conversion before the edit. Nudging the audio along in the timeline works until the next drift and then stops working. HandBrake, Shutter Encoder and most transcoders will write a constant frame rate copy, and proxy workflows in the main editors often do it as a side effect. If you suspect a clip, check its properties: a maximum and an average frame rate that disagree is the giveaway.