During the closing number "Louder than words," an audience member in the front row can be seen wearing a face mask. This is because the film was shot during the Covid-19 pandemic. The mask is only visible in short from behind the audience.
At around 1:04, when Jonathan is in the focus group, the reflection in the window behind him of the woman leading the group is *not* a mirror image of her as it should be. Depending on how the reflection was filmed, she either used the opposite arm in the reverse angle or the visual effects team forgot to flip her when compositing the shot.
In the brainstorming session, the list on the whiteboard contains the item "Santa Cruise," which is likely a misspelling of "Santa Cruz," considering the subsequent items on the list were locations.
In the beginning of the movie, Michael is complaining about singing a few lines before being stopped, but in the end, Jonathon songs about Michael not being able to sing when they were younger.
The prison barge didn't locate to New York until 1992.
In the sequence early in the film at the Strand, a number of books not yet published in 1990 are visible on the shelves. For instance, two books by Riley Sager ("Final Girls" published in 2017 and "Lock Every Door" in 2019) are visible at the top of the screen at the 8:37 mark.
The diner Jonathan works in has a beer tap for Redds Apple Ale, which wasn't released until 2013.