Factual error: It's not just the date of La Boheme they have wrong. Pirates of Penzance didn't premiere until 1880 and it was the apprentice who found love with the Major General's daughter and not, as stated, the Pirate King.

Continuity: When Charlie and Leopold are walking home after they go to the bar, Leopold says it is after midnight and they should go home. The next scene is back at the restaurant with Kate and J.J. When the camera is on J.J., you can see his watch and it says it's 9:30.

Factual error: The morning after Leopold and Stuart travel to the present, the two of them are talking about why Leopold followed Stuart to the bridge. Leopold mentions that he was afraid Stuart might be Jack the Ripper, whose first murder was in 1888, twelve years after Leopold's time.

Factual error: In the begining at the bridge ceremony, the band plays Manhattan Beach by John Philip Sousa. However, Manhattan Beach was composed in 1893.

Plot hole: Stuart establishes that there is only one of you and that when you travel through time you can repeat things, but it will be you and not your double that does it. But, at the end, when the Duke is going up the stairs and the uncle says "Where have you been...", the Duke is wearing the same clothes as the first time. Yet we see that he goes directly from the time portal to his house in the ballroom raiment of the later scene. Where did the old clothes come from?