Throughout the movie, Kenneth Calloway's Datsun alternates between being freshly painted with rear window louvers and having visible patina with a plain rear window.
In the beginning of the film when Jeff Garlin and Aubrey Plaza are having Chinese food at the table, Jeff is seen eating food with a fork, but as the scene goes on he is eating with chopsticks.
When Jeff is giving Arnau the get-lucky pep talk outside the motel room Arnau's collar is down/up/down between shots before Jeff actually turns it up after giving him his sunglasses.
Darius goes back to 2001 to prevent her mother from dying. With no way back to her present time of existence, she will cause a time loop. if she prevents an event required for her to travel back in time for to begin with, she will just cause the time-line to continually keep defaulting back on itself.
Jeff gives Arnau the "Are you gay?" talk. Jeff re-shapes Arnau's collar BUT between walking into the room and sitting on the bed, the shape of Arnau's collar inexplicably changes.
When Kenneth calls the instrument a zither he is wrong. It is a dulcimer.
While there is a period of seven or eight weeks in the summer when the entire Boy Scout camps is in use by lots of Scout Troops, camps are used throughout the year by individual Troops, Scouter Training Events, Order of the Arrow events, etc. Even if the camp had no one "booked" during a particular time, most camps have a full time Ranger that live at the camp to ensures that people like Kenneth would not be able to use the camp as extensively as Kenneth did.
When Darius is backing up the van to pick up Kenneth during the laser robbery, it is obviously a shot of the van driving forward (used a few shots later), but run backwards - exhaust goes into the tailpipe.
When Jeff goes to Kenneth's house the first time, Kenneth is not at all surprised that the guy knows his name and where he lives, even though the ad only had a P.O. Box.
When Kenneth and Darius are at the camp and Darius asks what kind of musical instrument Kenneth has, he responds that the instrument is a zither. The instrument they are using is a mountain, or Appalachian, dulcimer. Mountain dulcimers do fall into the general zither category, but typically when people refer to zithers they mean instruments that have shorter scale lengths and many more strings, or courses, than mountain dulcimers. Calling a dulcimer a zither is like calling a viola or cello a violin simply because they fall into the general "violin family" category.
When the ad for time travel is shown in the magazine office, the ad below the time travel ad is for a 1979 Hornet Wagon. AMC produced the Hornet from 1970 to 1977.