When traveling back into the past, we see visuals from WWII, the Civil War, and Indians attacking a wagon train; these events are out of proper sequence. The Indian wars took place after the Civil War and should appear before it when traveling back through time.
If the researchers were truly monitoring past history on screen they would be watching scenes in full color, not the black-and-white of the stock footage the filmmakers used.
At 1:00:40, the diamond Stanton picks up off the ground is a cut diamond, not a diamond in it's natural uncut state.
When ever the group goes back in time, past events are displayed on the monitor. What are supposedly reaction shots of each character are actually just still images of each individual taken from earlier scenes.
A lot of run time padding used in this film. The second time the Time Ship rotates, everyone is plastered against the wall, there is a cut to the monitor featuring historical events and the actor's faces superimposed over them. This segment runs well over three minutes.
When viewing the time periods as they pass on the viewing screen, there are superimposed shots of each of the characters that serve as their reaction shots. These in actuality are still shots of the actors taken from the segment in which they were speaking to the alien leader.