The pimp at the beginning of the 1977 exploitation flick Fight For Your Life is shown to be wearing loveheart boxers after Kane and his buddies steal his car and his clothes.
Murtaugh from Lethal Weapon 4; when directed by Riggs to strip to his boxer shorts to distract a madman wielding a flamethrower, Riggs asks with a laugh "Are those little hearts?"
Agent Simmons (played by John Turturro) in the Transformers live-action movie found himself handcuffed to a lightpole wearing some gaudy tropical boxers. Mikaela scoffs at that; the agent is too busy plotting revenge to be embarrassed.
Just standard striped boxers but played for laughs in Help! when Ringo is strapped into a Mad Scientist molecule-enlarging machine to remove a stubborn ring. All his other rings but that one drop off, then his zipper undoes itself and his trousers drop.
Rio: As Marcel is parachuting down with his henchmen hanging onto his legs, his pants come down showing his underwear is blue with ducks on them.
In Disney's Robin Hood when Lady Kluck is beating up Prince John's guards, one of them grabs her waist and pulls. Her bottom half slides down, revealing purple polka-dot underwear◊.
In Nerve, Vee is wearing panties with a cartoon panda face on the butt. When she strips down to her underwear to change into the fancy dress, one of the watchers comments on this and wonders how old she is.
Rita from Flushed Away is revealed to have blue-and-white striped boxers during the scene where Roddy accidentally pulls down her pants while escaping from the Toad and his henchrats while using the sewer gate's main cable as a zipline. Rita then proceeds to use said cable as a belt shortly after escaping.
Other women's underwear
In Anger Management (the movie, not the TV series), there is a scene in which Heather Graham has a cameo as a beautiful but psychotic woman. She invites Adam Sandler into her home (Sandler's therapist, played by Jack Nicholson, has set this all up, but that's not known at the time) and attempts to seduce him by appearing in nothing but Boston Red Sox fan lingerie (with the bra cups resembling baseballs, as I recall) and then starting to peel off the bra. Sandler nervously tells her to stop (he has a girlfriend), but this causes the woman to assume he thinks she's fat and to very slowly have a nervous breakdown, culminating in her screaming at him, throwing a chocolate cake in his face, and throwing him out into the rain.