First, as I age and also whenever I am playing couples golf with my wife, I find we are often quite content to quit after 12-14 holes, so it is becoming more common for me. What is surprising is just how strong the pull to play 18 holes is due to tradition. When I was younger, and actually amazed that someone past 60 something could still enjoy golf, I had a senior golfer take me aside to describe his typical golf round. "At 55, I still played 18, at 65, I played 9 to 12 holes, at 70, I played 9 holes, at 75, 6 holes. At 80, I play three holes, go home, make a drink, finish it and nap until dinner....."
As to quitting for boredom/anger/apathy/bad course, I can recall a few.
We were vacationing in Hot Springs, AK many years ago, and decided to play golf, and decided to walk for exercise, even though it was very hilly and over 100 degrees. The course was nothing special, to keep this on topic. I remember telling the girl behind the counter we were walking and she said, "You folks crazy!" She was right.
In Florida 7 years back I saw the former Jackie Gleason site Turnberry open for about 12 bucks on GolfNow and figured I would play it as a former tour course, remembering Jack's 5 straight chip ins on the final five holes. Only problem was, the course was in such bad shape and nothing really special we quit at 13, so I never had the chance to duplicate that feat.
I hate to say this, because overall I like the idea of the course, but it is just too hard for me, but I have quit Stoneridge east of St Paul (a Bobby Weed design) at 14(?) twice because it was a) getting dark, b) beating me up, and c) unfortunately, the routing got close to the clubhouse there, before playing away and then back from 15-18. I wasn't having fun, and it was clearly my best chance at shortcutting the round, so I turned left other than right to head back in.
Until that first time, I had never concerned myself with "giving an out" in the routing, and still think maybe the proper response ought to be designing an easier course.