When Freddy gives John Booth three more days he marks it on the calendar in red and the square he draws is not even, later when he crosses the day in the calendar the square drawn is perfect.
Peter's hair position changes when he says the famous line, "Did you get butt-fuc**d in prison?" and after John replies "It only hurts the first time".
When JoJo is dancing in front of John to Salt n Pepa's "Whatta Man", John gets up and stops the tape player. A minute later, he presses play on the tape player again, and the song comes in, however it is almost 4 bars rewound, and plays the same lyrics over again that has already been played, though no one rewound the tape.
The calendar on Freddy's wall shows January 1 as Monday (January 17 is Wednesday). This can't be if the events in the movie take place 5-6 years after Emily's death in 1988 (John Booth was in prison for 5 years). January 1 does not fall on a Monday in 1993, 1994, or 1995 (the year of the movie's production).