(at around 46 mins) When John returns from the woods, he opens the driver side door of his truck. The window changes from open in the shot from outside the truck, to closed in the shot from inside. Then, when he drives off, the window is open again. This was possibly done intentionally to avoid reflections of the camera crew, but it is still a continuity error.
During the basement scene, the duct tape changes the way it sticks to the clothes several times.
While there can be "75-year-old Scotch", whiskey including Scotch varieties only age in the barrel and once bottled that age is set. So, if you have a 12-year-old malt it will always be 12 years old, even if you keep it for 75, 750 or 7500 years. A 75-year-old Scotch would be an extremely rare and exotic find, not something you would be likely to find at a garage sale.
In the opening scene of the movie, the 14,000-year-old man is starting a fire (presumably during the early Holocene) using a bow strung with twine that looks pretty modern and was clearly machine made. However, the opening scene of the movie is not set thousands of years ago, but is a flash-forward to the penultimate scene where John is hiding out in a cave after fleeing his home.
Several moments with dialogue not matching mouth movements at the beginning or end of a shot.