Patrick, One fundamental mistake I can cite is when the designer places a bunker on the inside of a dog leg, tempting the player to carry it, only to have the area beyond the bunker unplayable. I won't mention who does this repeatedly because I will be accused of bashing. But, in general I agree we all learn more from mistakes than successes. If we can learn from others mistakes, we are one step ahead.
When I'm on a golf course and everything just seems to work properly, designwise, I'll catch myself not thinking about my critical eye because any criticism is barley noticeable. But, when there's a note out of place, and I become aware of it, my mind perceives it as a mistake. A great example of that happened to me on my first visit to Prairie Dunes. After playing the 12th those trees must have sparked something because I realized I hadn't thought one iota about the gca other than to interface with it. Standing on 13 tee I noticed these bunkers on the right that were not in character with all the previous bunkering. The next morning I went out of my way to meet Stan George the super and I asked him about those bunkers. he told me his predecessor had added those. When I met Doug Peterson(sp?), Stan's Predecessor, I asked him why he built those bunkers there. His response was as perfect as a response could be. He said his members were constantly losing their balls in that area, so he built the bunkers. Now, it may not be the best justification for their construction, but since it was only that spot, I accepted the answer and do not think less of them, or Prairie Dunes, for it.
That's what makes this DG so helpful. If someone can articulate why the mistake I perceived is not a mistake, I'm not so married to my opinion to not be able to see the other side.