Why does video load unpredictably? I'm having trouble trying to motion track a video, because it loads a frame ahead or behind depending on whether I play or seek through it.

As an example: I load my .blend and then seek to frame 70. Then I open it again, but this time, I play the animation until it reaches frame 70, then seek there. All of the trackers are now off.


This sort of error isn't good when I'm seeking and playing a lot, as it causes my tracking to look early or late. Is there a way to fix or work around it?
It turns out that this isn't a bug. I found that I have to check "Proxy/timecode" in the movie clip editor and set "Use timecode index" to "Record run." Now it works fine.