I usually record screens on my second monitor using the 'active desktop region'. Now I know all about switching screens with F8, but the 'active region' is clearly visible and selected on my second monitor, not on my other screen. Yet, upon forgetting to switch screens in the very beginning, Action records an invisible, non-selected region from my first monitor, and while it does (thankfully) remember the active region upon start-up, it doesn't seem to remember what screen I used.
So although the active region does turn red upon recording, apparently it doesn't per se record this region at all, and the only way to find out that the wrong region on the wrong screen was being recorded all along.. is upon stopping the recording, which of course is far too late.
(Also, as a sidenote, whenever I record MP4's for longer than 45-60 minutes, these won't work and end up corrupt and apparently unfixable. No issues with AVI so it's not life-threatening for me, but still.)