When the old man gets his head chopped off in the tunnel he clearly screams long after his head has been detached.
When Frank flees the concert with the money, his lock-box clearly contains no more than $50 in ones. How ever, one could argue that he had been interrupted by Reagan before he could fill it.
At about 1h 29 mins, when Gus turns the chainsaw on, it sounds as if it's turning on, but the chain visibly fails to move.
The US flag behind the stage is an old 48 star version.