My two favorite encounters from my Return to the Isle of Dread convention game:
Planned: a Gamma World death machine was located in a blasted crater, which was littered with magic items (the only thing remaining after the robot disintegrated opponents). It would not leave the crater, but if anyone climbed in it would begin to target them (random secret 1d4 roll for number of rounds for the shot to get off). Getting his was instant death. Centuries of powerful fools having tried to take on the machine, or steal the items, meant that there was lot of randomly generated loot to gather. Would PCs brave the crater in hopes of winning something way OP? Yes! Did some of them get disintegrated? YES!. Did any make it out? One -- because two went in from opposite sides, hoping it would confuse the death machine so much that it would not attack at all. Instead it rolled randomly who to disintegrate.
Random: When doing hexploration, I am big on using random encounter charts, and trying to make the results interesting. So when the party settled down one night I rolled a ghost encounter. I had the PC on watch witness a spectral young woman run to a nearby cliff's edge, look longingly out into the water, and then throw herself from the precipice. I expected the party to shrug and move on, and instead they spent a good deal of time getting down to the rocks and searching, so I decided that the young woman had actually fallen in love with a triton and the "engagement ring" he gave her was supposed to transform her as she leapt into the sea, but the not-Ursula hag he had gotten it from had tricked them and now the girl was a ghost and the boy was a tortured soul. The party found the ring in the rocks and used magic to learn the story, then killed the hag who lived in a nearby sea cave.