Up in Smoke (1978) review

The original 420-friendly movie (if you don't count Reefer Madness ironically): oblivious Pedro (Cheech Marin) picks up runaway Anthony Stoner (Tommy Chong) thinking he's a lady (which he's not), they smoke dope, wind up in jail, get released and stumble through a series of misadventures - including driving a van made out of marijuana and then entering a Battle of the Bands contest - while evading the persistent Sergeant Stedenko (Stacy Keach).  It's juvenile, ridiculous and exclusively for people who find pot humor funny (which I admittedly do not) - and at least 35% of the dialogue (written by the stars) consists of the word "man" - but I admire its chaotic energy and have to admit it's a hugely influential comedy.  To me, the most interesting piece of trivia surrounding it is that Marin's father Oscar was a police officer (for the LAPD) and his uncle was a detective ... so this could be just a bit of youthful rebellion, perhaps?