What was the largest in-game surprise you have experienced?
I try to be careful with the word “surprise”, because a big enough surprise is usually actually a swerve. If a surprise makes you think “wow, that’s really clever, and looking back I can see the foreshadowing that led to it and I am satisfied with this conclusion”, that’s a twist, and that’s great. If it makes you think “there’s no way I could have seen that coming, and I suspect it happened only so the DM or writers could shock people”, that’s a swerve, and that’s bad. You shouldn’t consider it a surprise as much as a waste of time at best and an insult at worst.
I suppose the biggest in-game surprise I ever had as a player was the first time I spontaneously came back from the dead. This was the post-Ragnarok campaign where the players were low-level gods. Since most of the gods had died, it hadn’t occurred to me that gods couldn’t die. But when I died and the flowers started crating a new body for me, I saw how the DM had made it clear that our powers included a sort of rebirth. It turned death from a mechanical problem into a narrative one because it stopped us from progressing while we waited a week and dealt with the repercussions of acting less quickly than we’d intended. It was foreshadowed appropriately, it caught me off-guard, and it changed how we played the campaign and how the setting dealt with us.
Since then I’ve died more times in campaigns than every other player combined, so some sort of “rapid resurrection” mechanic is always on the table, at least for me. I’m not terribly lucky (re: rolling 5d6 for character creation).