When Mattie enters her apartment at the beginning of the movie, she is wearing blue jeans. Then, while reading in bed, she is wearing light brown slacks. During her walk from her bed to Izzie's room, she is wearing the light brown slacks and suddenly has blue jeans on again.
When Mattie is at the college before going to Josh's apartment
her hair is straight in pulled into a pony tail, when she arrives at the apartment it is loose and curly. Her shirt also changed from light blue to a white shirt with "Nothing Rhymes with Oranges" on the front.
(at around 1h 13 mins) When Mattie arrives in the servers room, the PC unit it is black with 2 red tapes near it. She turn around and in the background the PC unit it is white with 2 red tapes near it. Then black again. The position of the chair near the table it is changing.
The movie is supposedly set in Ohio. The telephone area code (818) for the rental office of Josh's now vacant apartment is actually for the San Fernando Valley, California (and surrounding areas.) Furthermore, Zeigler's Columbus Zip Code should be 43201 (not the made-up 41273) and 7th Ave street numbers only go as high as the 3600's (not 13124). Plus the actual street is not full of multi-storied apartment buildings designed in the Eastern Bloc brutalist architectural style. Dexter's Weymouth Dr. street address (from his bounced check) is fictitious, but its Zip Code (44101) is indeed in Cleveland.
The goof item below may give away important plot points.
The cable that Josh takes into the other room with him is a Parallel Cable but the one used in the hanging is a UTP Network Cable. Furthermore, Mattie refers to the cable as "My boyfriend hung himself with a phone cable."