Last night I had a huge issue with a comp I was working on. I had isolated a flag and stabilized and keyed it. Once this was all done in the 1200x800 comp, I wanted to output a smaller window with just the flag in it. So I cropped the comp as usual in the viewport using the region of interest and then went to render a version with the output settings set to crop using region of influence. As it started to render I noticed the flag jump to the top left of the smaller comp, so I stopped the render. I turned the region of, but now without any other keyframes the flag that was static in the frame jumped to the left and up, then back to it's original position. I noticed that the Ram Preview was obviously using the Global cache so I tried to purge it but nothing I did would stop the glitches. I eventually emptied the cache and restarted. I think this fixed it but it was weird enough for me to be very wary of using region of interest in the future.
I'd just nest the original comp in a new comp of the proper H&V resolution and poisition to taste.
That was the first thing I tried. This seemed to bring the weird jitter ing placement to the new comp. I really think it was trying to use the cache which had the weird movement even in another composition. Of course I doubt I'll see this behavior again but not great when you are in a crunch