The clock in Justin's room reads 11:56 p.m. for longer than a minute over the course of two scenes.
His clock says 1200 when Justin hears his dad arrive and runs to the window. It says 1156 a minute later as his dad gets strung up.
In the first scene Justin appears in his commander space suit, it's completely reflective and shows the camera crew in the reflection. In the next few scenes, the space suit appears cloudy and covered in fingerprints indicating that the costume crew used a substance to make it appear cloudy to be less reflective.
When the teacher tells the class to turn to page 93 of their assignments, the textbooks are all titled "Dictionary." This appears to be the 1990 McMillan dictionary. The student practice section is 48 pages.
One of the envelopes to Santa can be seen to have no sender, but is returned to Justin's mother by the postman.