Say you need to replace low-res footage with a new, high-res version while maintaining pixel accuracy. First, create a new layer. Stack the new footage of top of the old.
In Final Cut, right-click on the top layer and choose Composite Mode>Difference. In After Effects, use the layer's drop-down Mode menu.
Your footage will take on an inverted and somewhat psychedelic appearance. But here's the great part: when the two versions are perfectly aligned, they will cancel each other out and your Viewer will become black.