Continuity mistake: In the second scene, when Cobb is taken to Saito (Old Man), Cobb's totem is kept on the table with the longer end down. But as the PoV changes to Saito's, the totem keeps changing position with the shorter end down. This happens at least four times. (00:01:45)
Revealing mistake: When Bobby's wife is dragged to her knees by the chain, you can make out the shape of her knee pads under her trousers. (00:41:10)
Continuity mistake: After Teddy takes his dead daughter's shoe off, its position changes between angles.
Continuity mistake: The last two pictures on the left on the wall going up the stairs change continually throughout the movie, at least 10 times. (00:24:35 - 00:26:35)