L.A. could never forget about Dr. Dre ... the legendary rapper surprised fans at Thursday night's FireAid benefit concert by joining Anderson .Paak on stage!

Green Day opened up the benefit concert and were quickly joined by Eilish for a performance of the band’s “Last Night on Earth” from their 2009 album “21st Century Breakdown.” Eilish departed the stage after the duet and Green Day continued with “Still Breathing” and “When I Come Around.”