Johnny throws the television out the window next to the front door and it appears to land in the alley where he and his friends had tossed the football around. For this to be possible, the front door would also have to open over the alley, leading nowhere.
Freshly-lit candles are beside the bed during the second love scene. However, Johnny and Lisa had been drinking for several hours before they went upstairs, and they were supposed to be so drunk they could barely stand.
In the scene with Peter and Mark on the roof, the blue box on the wall keeps changing position. In the shots with both Peter and Mark on screen, the front of the box is shown (easily seen by its yellow/golden decoration) and in the close ups of Mark, the back of the box is shown.
At the coffee shop, Johnny, Mark, and other customers don't pay for their orders at the counter. However, as Johnny leaves, Mark says to him, "It's on me." Presumably, at this shop, customers are supposed to pay before leaving. Mark is not shown leaving.
The pistol that Chris-R uses to threaten Denny is clearly a BB gun, as evidenced by the hole in the top for the BBs, and by the circular plug in the butt for the C02 cartridge rather than a rectangular opening for a magazine. However, Chris-R is not seen to fire it, and he uses it to good effect for intimidating an unknowing victim; he may have deliberately carried an imitation firearm for this purpose, perhaps due to its low price and easy availability, or in an attempt to escape prosecution if he were caught carrying it (carrying a real pistol is a serious crime in California if the person lacks the appropriate license or has a felony record).
When Johnny and Denny throw around a football while talking on the roof, one cut makes it appear that the ball is in the air for several seconds, even though they are only standing about four feet apart.
When Chris-R stands in the doorway leading to the roof, the camera is positioned behind his feet and pans to the right. When it pans right, the wall has no "back". The roof entrance goes from having four walls to three, then back to four.
The second floor spiral staircase has several steps that must be climbed before descending the main stairs. There is no logical architectural reason for this; the most probable reason for the extra steps is to allow characters to appear to descend the main stairs without the need for an actual opening in the soundstage floor, which would have been more complex and expensive to build.
Obvious replica gun - when Johnny shoots himself, text reading AIRSTRIKE 240 is plainly visible in close-up on the side of the pistol he uses. The Airstrike 240 is a popular Airsoft replica of the Beretta 92FS.