There's a trick for those side-by-side stills without a flicker. If able, go cross-eyed focusing between them to converge upon a 'third' image. If you've done stereograms, you'll know. Differences will show up easily as hazy and not so static if you gaze around a bit.
This is also related to how horrible eye witness testimony can be. There are good demonstrations of change blindness on Youtube. The human brain is incredibly awful at visual recall like this, regardless of advanced warning to concentrate. The longer a delay, such as by inserting a black frame in between, or diversion, the harder it is to spot differences between scenes. Here's a classic:
"Test Your Awareness : Whodunnit?"