I'm surprised no one has mentioned a Scotland trip! My first course was Prestwick and I recall one of my fellow architects stopping me during line up to tell me I was aiming 50 yards left of target, to which I responded "I don't think so." He also noted that an 8 iron wasn't enough club for 190 yards.....again, I didn't think so, and it turned out I was right on both counts, LOL. Many grumbled but I caught on to and liked the bump and run game.
In the states, while still working for Killian and Nugent, who did pretty predictable 2, 3, or 4 mounds behind each green and 1 or 2 sand bunkers, playing PGA National by Fazio (at that time) showed me how much more there could be to shaping, with his grass bunkers, lead in bunkers, variety of slopes steep and otherwise, shadow creation, etc.
Having played Medinah no. 3 in one of my first rounds, for a long time I took that as sort of gospel as to how great courses should be. I guess it was kind of a celebrity crush. At age 15-22, I was determined to design courses like that, with few bunkers but making them really deep. I gradually realized that the trend was more bunkers but not as deep, for a host of reasons. I also learned that M3 wasn't really highly regarded in all architectural circles.
The first two learning moments were courtesy of ASGCA.....the last, courtesy of Golf Club Atlas......