When Frank goes to visit his friend Sam at the steel mill, Sam's totals about the load of copper tubing are incorrect: "18% zinc, 42% copper, 38% tin, and one percent I don't know." This adds up to ninety-nine percent.
When Frank is with Joe Gaggs at the restaurant in the booth, Frank looks straight ahead to see Jessie at the counter. Yet, when Frank gets up to pay the tab he has to turn to his right to go to the counter; therefore, the line of sight is inconsistent, and there is no way Frank could have seen Jessie from where he was sitting.
Late in the film, when Frank gets out of his car to plant the bomb at his bar, the Green Mill, the lights used to illuminate the scene are clearly visible in the car door as the door's angle changes. When he opens the car door to get back in, the lights are visible again.
As Frank reloads his pistol in the final sequence, the slide of the weapon is forward, not locked back, as would be normal in a Colt semi-automatic, magazine-fed pistol. But if you count the shots before the reload, he only fired five shots, not seven, so the slide would be forward, since there was at least one round left in the magazine.
During the L.A. heist, the device intended to remove the key-lock assembly from the door leading to the vault is screwed in (undiscernibly) and jerked back without removing the lock, i.e., having no effect whatsoever. In the next shot, the lock assembly is gone. (Maybe it took more than one try.)
The goof item below may give away important plot points.
Incorrectly regarded as goofs
While it may seem inadvertent that Frank left his keys in the door of the Green Mill after planting the bomb, this was consistent with his plan of destroying everything that he believed Leo had given him: "home, car, businesses--I own the paper on your whole (bleeping) life." e.g., the house he bought for Jessie, his newly adopted son (David), Rocket Motors (Frank's cover business), and of course, Leo and all his henchmen.