What was the largest in-game surprise you have experienced?
Wow. I need to think about this one.
I think it would have been how well the flashback episodes worked with my kids.
I had done flashbacks during the run of the Dragon and the Phoenix campaign for Buffy, but it never felt like it did with my sons playing Come Endless Darkness for D&D 5.
I mean I knew it was going to work and work well, but I had no idea that the kids were going to enjoy it as much as they did. It was so satisfying that I want to figure out ways to do it in other campaigns.
Anytime my players can surprise me with their cleverness is also a great time.