Visible crew/equipment: When the elderly Elise returns from Richard's play and closes herself in her room at the Grand Hotel, the camera is briefly visible, reflected in the door glass as she enters the room. (00:04:00)
Factual error: When Richard Collier first arrives in 1912, the woman getting dressed in the hotel room is humming "You Made Me Love You," a song not published until 1913. It was first recorded by Al Jolson in June of 1913. The lady in the hotel shouldn't have known it yet. (00:30:00)
Continuity mistake: After Richard cuts his hair, it goes back to its original length at least three times during the movie. (00:30:10 - 01:24:10)
Plot hole: Richard eventually manages to travel back in time by removing all the modern items from his room. But what about the spotlights above his bed? (00:31:40)
Revealing mistake: When Richard first wakes up after travelling through time, he's supposed to be lying on a couch. Yet there is a wall behind him. It's blatantly obvious that it's the same shot from the modern bedroom, and all they did was change the lighting. (00:33:45)
Factual error: In 1912, Richard Collier gives young Arthur his ball, and above their heads, you can see the 1980 fire sprinklers in the ceiling. (00:40:40)
Factual error: When Richard first enters the 1912 theatre looking for Elise, he passes through a propped-open door that's equipped with a modern day hydraulic closer. (00:42:00)
Factual error: The 1912 hotel has frosted light bulbs hanging from the ceiling in some scenes. Frosted bulbs weren't around until the 1940s. (00:43:15)
Other mistake: When Arthur's father takes his ball away (just before Richard Collier signs the register), Richard is looking the other way while Arthur's father puts the ball somewhere behind the counter. Yet, when Richard decides to give Arthur his ball back, he reaches over the counter to the exact place Arthur's father placed it even though Richard never saw where it was placed. (00:54:30 - 00:57:10)
Continuity mistake: When he's tied up in the stable, Richard knocks down and breaks a lantern to cut himself free. He then sits directly in front of it to cut his bonds, and doesn't move to either side before standing up. When he does stand, though, the broken lantern is suddenly more than a foot to his left instead of behind him. (01:24:25)
Factual error: In scenes that take place in 1912 on the porch of the hotel you can sometimes see American flags in the background. The flags in the movie have 50 stars. The 50 star flag wasn't adopted until after 1959. (01:25:15)
Continuity mistake: During their "picnic" in the hotel room, Elise asks Richard what time it is. As he says that he doesn't care, his hands are both down and out of frame. When the angle changes, his right hand is suddenly raised to chin level, holding the champagne glass, with no time for a natural transition to that pose. (01:30:00)
Factual error: When Richard Collier encounters the young Arthur in 1912, Arthur is playing with an inflatable rubber ball. Balls of this style were not made until the 1960s.
Continuity mistake: When Reeves signs the guest register at the hotel in 1912, his signature is different from the one that he saw in the register that he found in the attic.
Plot hole: The watch that is presented to Collier by Elise which starts this entire story is never purchased, stolen or acquired in any way, but left behind by Collier. It is just there, which is a paradox in itself.
Factual error: When Richard and Elise are walking down a hallway, he asks when the acting company will leave. Notice the two doors they pass on their left. The doorknobs are modern replacements with large, shiny brass plates that reinforce the door for better security, commonly retrofitted during 1960-1980 in public buildings with older "skeleton key locks". They are seen during the 1980 scenes, but in 1912 they should not be there. Interior locks of that era usually had smaller plates, like the one Richard first sees in the occupied guest room.
Continuity mistake: When Richard Collier is leaving Chicago near the beginning of the film, he is seen driving past the John Hancock Tower. The sky is overcast and the clouds so low that the top of the building cannot be seen. In the next shot, however, skies are partly cloudy and the building can be seen in its entirety.
Continuity mistake: When Richard is first speaking into his tape recorder in his hotel room, there is a beige "touchtone" telephone in the background. When Arthur is using the telephone at the end of the movie, it has changed into a rotary dial phone (it is evident when you hear Arthur dial 0 for the hotel operator).
Factual error: In the final sequence, Richard Collier is shown sitting in a chair where, as others break into his room, we are lead to believe "he had been for days". Yet his hair is clean and combed, his clothing seems immaculate, and apart from some makeup, he shows no sign of any mental or physical problems.
Continuity mistake: Near the end of the movie, C. Reeve is seated at a table on the lawn, and C. Plummer approaches, with the intent of encouraging C. Reeve to leave town. As they are talking, the camera goes back and forth between them. Whenever the camera is focused on C. Reeve, he is enjoying either a full meal or a cup of coffee.
Answer: There are several things like this throughout the movie. It's a rather busy location.