Oh no! Ben Affleck did some damage last night at the Unicef Halloween party at the La Peer hotel Saturday night -- clearly breaking his sobriety streak that had lasted for over a year since ex-wife Jennifer Garner staged an intervention at his home in August of 2018.

Ben was masked in an attempt to hide his identity, but we spotted him amongst the crowd of A-listers leaving the party and TMZ captured him in a video showing the actor stumbling -- almost falling down -- as he grabbed desperately onto a waiting SUV after taking a selfie with a hot, young lady. He then walks -- zombie-like -- to her chauffeured SUV and jumps in the car with her. The pair returned to Ben's Pacific Palisades mansion for a sleepover.

Poor Ben! Remember -- one mistake doesn't mean it's over. Jump RIGHT BACK ON that wagon and keep your chin up!