I've seen it twice, maybe three times, always when I walked on by myself. Once it was offered to me and I declined.
When I first started playing, I got paired up with a real blue collar sort of guy, friendly enough, who pulled a 20-oz Coke bottle out of his bag, he took a few swigs, and dumped all but about 1/4 of it out. He filled it the rest of the way with Everclear. No exaggeration - I asked him if it really was Everclear. He admitted it was, that it calms his nerves, that he knew most doctors would say he has a drinking problem, but he just likes to get out and relax on a Saturday. It was about 7:00 a.m. at the time. He told me that once in a while he'll go to a more upscale course with bottle he had carefully removed the seal, fill the bottle about 2/3 with water, re-attach the seal, and try to get paired up with some "cocky rich dudes." He said he would make a scene of pouring his "cocktail" and offer some Everclear to the "rich dudes" who would think he was a drunk who was about to get blotto. After gulping about half his bottle of "cocktail" he'd get a bit loud and boisterous and obnoxiously try to goad the other guys to put some cash on the round, which they would think was easy money with how wasted he seemed. He said he usually took the other guys for enough to pay for his round at the nicer course. He could tell I was a beginner, he wasn't trying to get me to play for money, and I told him early on that I don't gamble anyway, so I don't think he was trying to pretend a bottle of his watered-down stuff was pure grain, I think it was the real deal. He must have had no liver left, because after 12 holes (bizarre little muni course that they expanded from a 9-hole course but never finished the other six holes), he still seemed about the same as he had before we started and was mostly finished with his bottle.
On July 4th this year, I played a rural country club that was open to the public that had attacted a few foursomes of good ol' boy golfers all in US flag shirts, one with boom box blasting "Proud to Be An American" on the back of his cart, all of them pretty liquored up before 9am. The foursome behind us seemed to be hitting the bottle harder than the ball which was absolutely crazy, since it was in the upper 90's before noon. On the 13th hole, one of the guys ended up puking out his guts next to a port-a-potty and looked like he was going to pass out. Maybe he refused to leave them or maybe his buddies were as dumb as he was and never thought about him, but he rode with them the entire 18 holes, instead of getting driven straight to the air conditioned clubhouse. Morons.
I'll have a beer or two sometimes on a round, but often will just stick with Gatorade or water. The one time I had four beers on one round of golf I ended up losing a bunch of balls, some of them probably not too badly hit, because I kept forgetting to concentrate on where they were going after I hit them and couldn't figure out where they landed.