Revealing mistake: In the scene where Harry and Lloyd first meet by running into each other, when it shows it in slow motion, you can see Lloyd brace for an impact when he's not suppose to know Harry is coming.
Revealing mistake: When we see the slow motion replays of Harry and Lloyd colliding, you can see that Lloyd's tooth doesn't touch Harry's head. In fact, their heads completely miss each other.
Revealing mistake: In the scene where the guys get the flagpole wedgie, we see Lloyd's coat being pulled up by invisible wires.
Revealing mistake: In the scene where Harry is going to Jessica's house on the waxer, if you watch carefully most likely you'll catch the last person who falls is wearing roller-blades like the kind that look like shoes.
Revealing mistake: When Jessica is riding in the principal's car they pass a gas station where there is a white late 90's/2000ish van. This film is set in the 80's.
Revealing mistake: In the shot when Harry and Lloyd are holding on the principal's car while riding in a shopping cart, the speedometer on the principal's car reads zero, while they are rolling forward with some speed.
Revealing mistake: When Harry and Lloyd collide, Lloyd's tooth goes into Harry's forehead. When he pulls it out, he doesn't even bleed, and it leaves a little mark. Also, the little mark from the tooth is gone by the next scene.
Answer: Because Harry and Lloyd had poured gasoline all over the car as well as let the pump keep running when they left. So when the hot ash of the store clerk's cigarette hit the gas fumes, it caused the explosion.
Bishop73