I'll give you my example, and then I'd like to hear about yours. Looking for examples of courses that you've played before that snuck up and bit you with a feature you didn't know existed in previous rounds.
A couple of weeks ago, I played 36 holes at the Yale Course (the 2nd and 3rd times I played the course.) I had played the course one other time two years ago. We walked the first two times we played the course. We carted it for the third round, since we were pressed for daylight (and a little tired from the hike over #18 mountain in the afternoon.)
Both times I walked and played #8, I missed the green either short or left. Extreme left is not a good place to be. Lost ball, unplayable lie, or extremely tough recovery shot are all possibilities from the pit of despair. Normally I do a decent job of looking around and inspecting the architecture for future rounds. The first two times I played this hole I did a poor job of fully assessing the situation for future rounds for a few reasons:
1) I struggled on the hole both times and it drained me mentally
2) The direct line from the green to the next hole takes the walking golfer off the back left of the green
3) The presence of the 9th looming in the near distance
Enter my third trip around the course. We tee off on #8 and I miss wide right and also pop the tee shot up a bit. I've got about 200 yards to the green, which is a hybrid for me. I tend to fade (I'm a lefty) my hybrid when there's trouble looming in that direction. I tell my playing partners, "There's no way I'm missing this one on the left side." I proceed to hit a shot long and to the right, and I'm mentally high-fiving myself.
Well, Mr. "excited about #9, so I never stopped to look around" has now discovered a new part of the course that he didn't know existed. I guess I should have realized there was something hiding behind the green that is so severely banked right to left, but it never dawned on me to take a peek. I find my ball in a massive bunker, facing a delicate shot over a high grass wall to a green that runs away from me. It was wonderful and horrifying all at once.
I flubbed the bunker shot, but got out of the sand. Hit the next one up on the green and it ran just off. Two-putted from just off the green and took my double bogey 6. It was still better than the disastrous 9 I made earlier that same day, but it was bad enough to spend a good amount of time thinking about where I'm going to miss next time I play that hole. I'll probably bump one up and leave it short.
Here's a few photos, though they don't fully capture what I've described
A view of the green through the dogleg. You can see the steep decline down to left side bunker. What you can't see is that left of the left bunker is an absolute disaster. My experiences over here are what drove me to the surprise bunker behind the right side of the green.
Here's another view of the approach from the fairway
Here's a shot of the hidden bunker and the wall of grass confronting the recovery shot. The green completely runs away from this shot
Here's a view of the green looking back on the hole. You can get a idea of how the green runs right to left (left to right in this photo) though I assure you all the greens at Yale are more severe than an amateur photographer can capture