Continuity mistake: When Paige is shown brushing her hair in front of the mirror, there is no background shown in the mirror behind her; it's completely black. We see this a few times while she's sitting at the dressing table, but then when she gets up to gather her things and take them to the kitchen, we see the room reflected normally. (00:43:25 - 00:44:30)
Continuity mistake: When Mazzilli is talking to the reporter in front of the school, he lights smoking firecrackers and throws them behind him. When we see him light the first one, it is at least three or four inches long when we see it in his hand, but much shorter an instant later when it hits the ground. This happens again with the next one too, but this time it's shorter when we first see it and slightly longer when it hits the ground. (00:54:25)
Continuity mistake: The FCC commissioner's limousine window goes up and down in between shots. (01:26:25 - 01:27:55)
Continuity mistake: When the teachers play the confiscated tapes of Harry, it's Christian Slater's voice, not the disguised voice you hear on the students' radios.
Continuity mistake: The entire 'typical American High School' wears the same clothes two days in a row, as a result of one day in the script which was divided into two days during the editing process.
Continuity mistake: When the guy in suspenders sneaks a cassette tape of Harry's show into the boombox, at the place where the students are dancing, the recording of Harry busting Deever is a completely different performance. You can tell because the word "Why" (from "W-w-why is that, Mr. Deever?") is much lower in pitch.
Answer: The first time I watched the movie, I kept thinking that Mark's parents were going to hear him and catch "Harry" in the act. When the girl was with Mark, Mark's parents pounded on the outside door; when Mark opened the door, his parents said they thought they heard him talking to someone. So, from outside the garage door, someone might be able to hear muffled voices but not the actual words. Why his parents cannot hear him in the basement when they are indoors lies in the props/scenery plus some inference. The numerous objects in the room (tapes, CDs, albums, guitar, drums, bongo drums, recording instruments, amps, etc.) indicate that Mark is into music - loud music - and electronics. He apparently was given garage/utility storage space to turn into essentially a studio for himself and a place to play his drums and music without disturbing his parents. The space has been sound-proofed - thick concrete walls, insulation, and cloth wall hangings to deaden the sound.
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