Continuity: When Cheech is turning over his car to pick up Chong you see him starting to turn over before he reach Chong. When it cuts Cheech has passed Chong before turning over.
Great sites
Mistakes
During the Battle of the Bands, when Cheech is playing his electric guitar it is not plugged up. Also, his voice is echoing through the stadium but he's nowhere near a microphone. See more...
Cheech and Chong's Up In Smoke mistakes
Revealing: When the cop on the motorcycle pulls Cheech and Chong over, one of the hippie girls is sitting in the front with Chong. Chong orders her to get in the back. She goes back behind the curtains and sticks her head out front. He pushes her back again. In a shot from the back of the van, you see Chong pushing her back, but you also see through the crack in the curtain that it is totally dark in front of the van. Since there is daylight outside, it shouldn't be totally dark.
Revealing: When Cheech and Chong are parked in their car smoking a joint, the car is filled with smoke. The smoke must come from a smoke machine outside as it is moving around very fast as if blown by the wind.