The Time Machine (1960)

Revealing mistake: During the big underground fight scene, the solid rock wall moves as George pounds a Morlock's head against it.

Grumpy Scot

Revealing mistake: George is underground fighting the Morlocks and kills several of them. As he leads the terrified and apathetic Eloi out and back up the ramp, one of the dead Morlocks very thoughtfully moves his legs out of way.

Revealing mistake: When the time-traveller enters the Morlock caves via the ventilation shaft, you can see (when the screen is bright) that the background of the cave behind the machine section is merely a painted screen.

Revealing mistake: As George comes to the year 1917, the camera shows that the buildings in the horizon are nothing but poor painting, especially on the wall around the archway. The two buildings right beyond it have distinct differences above and below the top of the archway.