When Mark is playing Doom II: Hell on Earth (1994), he is shown on the second level, Underhalls. When he leaves and a nurse takes over, the computer screen shown a few seconds later displays a room halfway through the first level. Additionally, the screen also abruptly cuts to a completely different room, indicating that the game footage was prerecorded and not actually being played in real time.
When Doug and Joey go into the storm drain, it's light out with no indication that the sun is setting. Two and a half minutes later, Doug comes out of the storm drain, and it's dark outside. Even with rain clouds, this should not be possible in Chicago.
When Mark plays Doom II: Hell on Earth (1994), he shoots the chaingun, but the sound effect played is the plasma gun.
During the rescue, when Dr. Ross is singing 'Take Me Out to the Ball Game' he sings "Root, root, root for the home team." Any self-respecting Chicago Cubs fan, as Dr. Ross is shown to be on multiple occasions, would sing that line as "Root, root, root for the Cubbies!" This version was made famous by Cubs broadcaster Harry Caray as he sang during the 7th Inning Stretch at every Cubs home game.