At 2:49, the leader of the Phantom Gang has a revolver in a shoulder holster on his right side (for left hand draw), but at 10:56 when he is seen in the hallway, drawing and firing the revolver, the shoulder holster is on his LEFT side (for RIGHT hand draw) and he's RIGHT-HANDED.
During Shemp's "death scene" after the poisoning, actress Christine McIntyre has a hard time keeping a straight face. (I suppose one could argue that her character was delighted over the soon-to-be-successful murder, but to me it looks like genuine out-of-character smirking.)
When Moe is running down the hallway after crashing through the door, he is seen with a limp. That is because Moe injured his foot in the previous sequence.
After Shemp knocks everyone out, he calls out "Moe!" and "Larry!" after discovering them on the floor, but he is looking at opposite directions.
After 'The Niece' chooses Shemp, Moe kicks him in the backside. When Shemp looks back he says "Don't you raise your hand to me" , but Moe had not raised his hand to smack him yet. Larry gives Moe an obvious look that they blew the scene.
When Delores describes the pills she has she says "sleeping pills, the permanent kind".
However when Shemp swallows the pills they don't put him to sleep permanently, he eventually gets up and he is fine.
A sequence has the Stooges running to a window in the study to escape, but the scene cuts to the Stooges trying to phone the police.
When Moe gets knocked out with the shovel by Shemp, he visibly just lays down on the floor.