Marc Maron -- Selfie With Obama ... In His Garage

One minute you're in a Beverly Hills mansion ... the next you're in some dude's garage -- at least, that's how it went for President Obama today when he stopped by Marc Maron's house to be on MM's insanely popular podcast.

POTUS hit up the comic's Highland Park home (near Dodger Stadium) to be a guest on "WTF with Marc Maron" ... which Marc still records in his actual garage.

No word on what they discussed yet (the podcast airs Monday) ... but Maron is known for being able to have some pretty "real" conversations with huge stars ... like Robin Williams, Louis C.K. and Ben Stiller.

After the show, the two posed for a selfie ... but Obama must've been in a hurry, 'cause he left without his coffee cup.

One man's trash ...