There are so many, but here are my favorite two:
Non D&D homebrew game centered around Mecha:
The plot consisted of the group investigating a young girl, Delta, who was engineered to be a perfect meca pilot. Sad little girl with long greasy black hair (kind of like River Tam, but this game happened long before Firefly).
A player had the idea of kidnapping her after a test run of an experimental Mech. His plan was to hide in a spider hole on the arena, and once the test is concluded, he was going to run up to the 30 foot mech (named Alpha and Omega), scale up it, place a charge of explosives on the pilot door, blow it off and yank little Delta to freedom and raise her like a daughter.
As he popped out of the hidie-hole, the proximity alarm of the mech went off. Delta got the instructions to terminate the intruder. The mech swiveled in place and lowered its gauss cannon and vulcan gun to the target.
I really gave him every chance. I discouraged him from the plan from the beginning, I even rolled to hit.
The rest of the encounter was described by what the other player saw: You see him skid to a stop and look up at the weapons pointed at him. A loud rumbling noise fills the arena as the vulcan cannon spat slugs the size of babies at your friend. Dirt and dust are thrown in the air followed by an explosion. When things settle, all that is left of him is a pile of stained, ruined earth.
The second is a good example of why DMs need to check the character sheets of players they are allowing to join a campaign mid way.
The encounter went something like this:
DM: "ok, the 12 orks get up from their tables on the other side of the tavern and draw weapons"
Player: "I pull out this little wooden box, throw it at the orks and say the command word."
DM: "ok, what's it do"
Player: "it turns into a skiff"
DM ". . . waits. . a . . . a skiff? like rowing in the ocean type skiff?"
The encounter didn't last long.