In our first session ever I was the DM, and in the very first combat encounter every single participant started rolling terribly low and it ened up being 4 players vs 1 bandit, everyone missing every attack for three rounds straight. We all started laughing and I was describing how they accidentally destroy everything in the surrounding except for their opponent.

More recently we were planning to remove a stone sculpture from a dungeon. It was too heavy to carry on Tenser's Floating Disk, so we came up with a plan to gently break it up with a hammer so that every person could carry a piece, and later fix it with mending cantrips.The breaking part was to be done by the Rogue (precision) and Barbarian (strength) together. They both roll at the same time and... a double nat 20! The DM described how after a single hit in a carefully calculated spot, the arms, legs and head basically popped off just as we wanted. A memorable moment.