When Capt. Sedman interviews Sanderson after the murder of Debi Rutlege he states that she is not listed in the directory. Yet when Kathy later goes through Sandersons directory to look for a pizza place there is an entry for Debi Rutlege which Sanderson has marked.
After the 'carefully' murder, there is an attempted murder on Kathy.
And somewhere later on, he is taken into custody and writes the words on the blackboard, all six: Remember Eventually Revenge Is Carefully Arranged.
But the Arranged murder never happens.
When Erica is shown playing Super Mario Brothers 3 on a PC, the moves she makes with the joystick as well as her keyboard tapping are inconsistent with what is shown on the PC monitor. In the next scene, she moves away from the PC but the game is still playing by itself.
Midway through the film Peter Sanderson and Frank Sedman are conferring with a computer whiz. When we see a full view of the table that the men are standing around it's not Sedman but Tom Skerritt's stand-in - his face is in full view.