I needed to simultaneously film the front and back of something so I can composite them together and show both at the same time. I thought it would be enough to just clap at the beginning of the recording and use that to align the tracks but I've found that despite that, they still end up out of synch and even more confusingly go back into synch further on. It may also be that the sound and video desynch in the footage recorded by my phone. It's a Samsung Galaxy S5 so I figure the hardware should be okay but maybe not the app (Open Camera)? The framerate is variable which might be related.
I'm still at the stage of using Shotcut to transcode clips from the footage so this is not occurring in Hitfilm (which doesn't seem suitable for quickly transcoding multiple clips of three synchronised videos (yeah, I am actually doing three, not too but didn't want to complicate my query).
Edit: maybe they don't go back into synch; maybe the audio stays in synch and the video goes out of synch.