At the beginning of the film, the monster closed after passing through most of the doors. But when the teacher was going to see what the students were doing, these doors were all open.
When Mike congratulated Celia and then turned and left, it was obvious that he had crossed a conspicuous mark on the floor. But in the next shot, recalling the scene before them, the distance between him and Celia was drawn very close, and he clearly stepped over the mark on the floor.
At the beginning of the film, Mike asks Sulley to wake up. You can see that there is an alarm clock on the right hand side of Sulley, but in the next shot, when Boo jumps on Sulley, the alarm clock is gone.
When Waternoose congratulated Suelly on winning the best moving scream award, monsters of different shapes surrounded Suelly, but just a few seconds after this shot, the monsters who hugged Suelly disappeared without a trace.
When Randall walked through the door of the screaming platform, the handle of the door was on the left, and it didn't take long for Randall to appear in front of the door again, changing his handle to the right.
At the end of the film, when Sulley and Mike met the CDA numbered 00001, many CDA monsters had the same number. Perhaps to save time, the number "00112" appeared countless times.
