I don't know if this qualifies as a bug, but it's unexpected behavior.

While I'm working on a project I usually do pre-renders at NTSC Draft quality to save time, then at the end of the project do a render to normal or Best NTSC quality. However, it turns out that when doing the final render to a new track, if there are sections which were prerendered as NTSC Draft, those sections will not be re-rendered at the higher quality. So you'll end up with good quality interspersed with poor quality. I experienced this several times, but never knew what was going on until now.