Continuity mistake: When Chong's father is shouting "we're not gonna have a family brawl," there is a big bottle of juice on the edge of the table by Chong. It cuts away for three seconds and then back to Chong. The bottle of juice has moved to the middle of the table. Also, Chong has poured juice into the mixture. He wouldn't have had time to do all that in just three seconds.
Continuity mistake: After Cheech has asked the girls if they need a ride he spots Chong hitchhiking. He then takes a u-turn to the other side of the road. At this time you still see the girls sitting in their original position. When it cuts you suddenly see a van has appeared from out of nowhere and the girls are suddenly walking to it.
Continuity mistake: The drum kit is a Ludwig amber Vistalite (see Keith Moon, who had a clear one, and Karen Carpenter, a blue one, neither had bottom heads) like John Bonham played in the Song Remains the same. The drum head on the kick drum in the back seat is on and off in the same scene repeatedly. They're a pain to remove, so it's physically impossible to do so and replace in the same conversation.