Some of my tracks don't show the video they should: I edited one track how I wanted, and some time later I found the whole video sequence had a starting offset that came from nowhere and ruined my editing. Since re-building proxies, deleting caches and restarting Blender didn't help, I fixed it by shifting the strip backwards with S.
Now I the audio is out of sync for no reason. But when I render it, audio and video are in sync again, but the video is shifted again. I really don't know why preview and render differ and why my tracks get shifted. It's incredibly annoying.
Just to try out, it imported the track as Clip into Blender (normally, I import them as movies directly into VSE) and copied the edits from the track. So I have two tracks with same position and clip showing the same video, but one added as Movie and the other as Clip. Of course, they are not in sync but one has several seconds in advance on the other. Contrarily to my expectations, the new track is not in sync with the audio either. Now I'm heavily confused.