(at around 1h 14 mins) When Orlock's car pulls up to the drive-in ticket booth.,the clock says 9:10. After talking with the two fellows at the booth for a minute or so, the car pulls away and the clock says 9:00.
The cost of an adult admission to the drive-in (posted on admission booth in later scene) is $1.25 for adults. But when Bobby buys a ticket, the change he receives from the ticket seller money is consistent with $1.50 admission charge.
(at around 11 mins) Exterior of Thompson home does not match layout of interior sets. When Bobby pulls up in front of house, front door is set flush in a wall that runs entire length of building; yet when he enters, there is windowed wall that runs along right side of entry hall that could not possibly coexist with exterior.
The soda bottle's position changes several times as Bobby lies atop the oil tank to aim his rifle; also, the level of the liquid in the soda bottle changes between shots from full to nearly empty.
While Bobby is at the gun shop stocking up on ammo, he's eating a Baby Ruth candy bar--that stays its full length between shots though Bobby is obviously consuming it. This indicates several takes of the same shot.
Bloody wounds, entry and exit, are on Bobby's wife and mother, but no such wounds appear on grocery delivery boy victim.
(at around 1h 28 mins) At the very end of the movie, the drive-in is empty the next day, except for the killer's car. This is illogical: the victims' cars should be there too.
When Bobby drags his mother into the bedroom after shooting her, she moves her head as he's arranging her "dead body" on the bed.
Bobby's check to the gun shop is dated late July, but when Orlok picks up a newspaper story about the supermarket massacre several days later, the paper carries a March date.
(at around 39 mins) As Bobby drags his mother's body to her bedroom, a camera shadow can be clearly seen moving up the wall and door to the left.