I'm not sure what I'm doing wrong but I noticed this on the first video I edited using PD12 (this is the second) and I thought it was a rendering issue. For some reason, my movies look fine in timeline during edit, but when I produce them into video files I noticed there are random segments where certain cuts have tiny black bars on the sides (similar to 4:3 on an HD display, but the picture is not stretched in anyway).

Here's my current cut (still have to edit/adjust audio, subtitles, etc). Look closely at the sides: https://www.youtube.com/watch?v=My-Lfd8OeSk

I was hoping this issue would go away for my second video but it's still happening. I produced the video as mpeg4 and .mov both in 1080 by 1920 and I'm getting the same results. I don't know why most of the clips are fine and why random ones have this tiny black bar issue. If anyone has experienced and solved this issue please help!