That's exactly what happened Sunday to luxury real estate agent Dwayne Henry of Westside Estate Agency, who had a little car trouble off the Sunset Strip ... and was somewhat rescued by none other than the legendary late night talk show host in the flesh.

Dwayne tells TMZ ... he took his 1953 Mercedes-Benz -- a classic car if there ever was one -- out for a spin 'cause of the nice weather and to beat the quarantine blues, and almost immediately ... he knew something wasn't right mechanically. So, naturally, he pulled over.

DH says the issue was so bad, he called a tow truck right away ... but between the time he called and the truck arriving, Jay pulled up with his wife in a Tesla seemingly out of nowhere. Sure enough, Dwayne says Jay walked over and asked to have a peek inside.

Of course, Jay is the owner of SEVERAL classic cars -- including an even older Mercedes-Benz than this one. So, it's no wonder he spotted this whip -- but wild he actually reacted.

Anywho, as Jay was fiddling around in there, the tow truck driver showed and tossed Jay a wrench so he could tinker some more. Eventually, Dwayne tells us Jay had the diagnosis ... a brake issue that was also screwing with his fluid. Unfortunately, we're told he wasn't able to band-aid it on the spot, but hey ... an official car workup from JL ain't too shabby.