Box Cutter - S4-E1
Continuity mistake: When Gus starts to walk away from where he killed Victor, he drops the Box Cutter, and we see it landing. However, after a cut to the next shot, its position has changed. It is no longer sitting on the line in the floor, and it is pointing 180° in the other direction. We can also see from Walt's shadow that the Box Cutter has moved quite a bit further away from Walt. (00:35:05)