The "Caddyshack" actor wasn't playing well at all at the famous Northern California tournament ... so when a fan offered him a mid-round shot of tequila -- he downed it, and the video is hilarious!!

You can see in the clip ... Murray was walking up the fairway at one of the holes when a woman leaned over the ropes with a plastic cup full of booze.

"Hey, Bill!" the woman screamed at him. "I got a shot of tequila for you! I know you'll like it!"

Murray walked straight up to her, took the cup, slammed the drink, threw the empty and kept on walking like nothing had happened!!!

Of course, Bill's known to clown around on the golf course -- especially when he ain't striking it well -- and his mid-round shot was hardly his only laugh-out-loud moment from the tourney.

On Sunday, he missed a putt badly ... but instead of letting it settle, he redirected it in and walked straight off the green!!

It's illegal, but funny ... and, let's be honest, that's the way golf SHOULD be played!