Continuity mistake: When Dennis pinches Mr. Wilson when Mr. Wilson is pretending to be sick, you can see Dennis pinch but he doesn't use his index finger. When it cuts he is using his index finger. (00:04:05)
Continuity mistake: In the scene where Dennis and his mother are on their way to Margaret's house, the lock on the passenger's side door is locked and then unlocked when the camera view changes. (00:11:40)
Continuity mistake: When Joey is dragging the perambulator you see him dragging it while walking backwards. When it cuts he is dragging it behind him walking forward. (00:20:00)
Continuity mistake: When Dennis is in the bathtub you see some toys floating around in the tub. When it cuts from above him the toys are gone. Instead the whole tub is covered with foam. (01:28:20)
Continuity mistake: When Dennis is in Mr. WIlson's bathroom after his bath, pouring mouthwash in his nose spray, Dennis has toothpaste all over his chin and mouth. When they show Dennis from the side, it is gone. From the front it is there again. This happens several times in the shot.
Continuity mistake: While Dennis is stoking Switchblade Sam's campfire, he flings an ember that falls down the back of Sam's pants. The smoke (and later the flame) are coming from the center seam, yet after Sam puts the fire out the hole is a couple of inches to the right.
Continuity mistake: When Switchblade Sam is in the water and Dennis lowers him the rope, he barely gets it around his ankle. When he gets back onto the bank, it has somehow become neatly tied around his waist (obviously for the stunt that follows).
Continuity mistake: When Dennis climbs onto Mr. Wilson, who is pretending to be asleep, his left shoe is untied, but when he leaves and runs home it's tied again.
Continuity mistake: During the bathroom scene, Dennis pulls the cap off the nasal spray bottle and throws it. When the scene cuts to him holding the mouthwash, the bottle is sitting with its cap on, and immediately after the cap is off again.
Continuity mistake: In the bedroom scene, Dennis' overall buckle is normal. But after he pops the cap off the aspirin bottle, it's pushed all the way to the side. The next shot shows it as normal again.
Answer: Because of what George said to him in the house after he was robbed. He hurt Dennis' feelings so he ran away.