In most D&D games, it is a team adventure, but every now and then a player wants… no demands a solo adventure for their PC.
On a more direct involvement, we had an assassin (1e) who would bug the DM endlessly for over a month to get an assignment from his guild.
The DM finally gave in, and a contract to kill a local tax collector was offered. This old human male lived in a mansion at the edge of the city. What the assassin did not know until it was too late that the tax collector was Geryon in disguise.
Surprise back stab attempt and many magical weapons (critical miss, damaged/destroyed weapons) later, and down to 2 or 3 hit point, the assassin grabbed its last weapon, a vorpol blade and score a natural 20 to hit!
After that, the player was OK for a week before wanting another contract. He enjoyed it too much. Sure it cost him all but his last magical item.
So the DM, in the next session and before they group had a chance to get any replacement weapons, the group was in the wilderness and we had to camp for the night. A thick mist entered, obscuring all. We all awoke to the smell of smoke and brimstone. The party left their tents to discover they were now on a tall and wide stone wall, and the assassin’s tent was not here.
As the smoke lifted we discovered we were actually in what would be called the royal box of an arena and the assassin (and his tent) was twenty yards away in the middle of the arena.
The assassin (and the group) were confused as to what was going on.
As more of the smoke lifted, we saw demons and devils filled all the sits in the arena.
A huge horn sounded and thru the only gate into the arena a fully pissed off Geryon entered.
“So little man, you were able to kill my avatar in the prime plane with that weapon of yours. How lucky do you feel for a rematch… on my HOME PLANE of hell!”
While the players all laughted out loud, the player/assassin groaned. “Oh sh!t”.
After that session, no player decided to bug any DM about such quest(s) that is only for their PC to do.
Ive got a player who goes out of his way to make characters that are worthless. The character he was running when I met and joined the group was a dwarven battle priest. Who refused to enter combat if his half dozen buff spells were not up. He got me killed three different times cause some oponent dispelled his character and he retreated. The newest character in the game im running is a minotaur priest (monk) of a god of pacifists…
The group started a western game a while ago; his 3rd character died two sessions ago. The one he dropped on us Friday night is a prospecter… In a game where most of the characters are city slickers.
Id love to boot his ass but he was with the group a decade before I met them. Or more even.
I ran a game where a merchant had a special going out of business sale. He had a bunch of fairy touched Zonko products that included:
Potion of fly: it summoned a fly
Ring of 3 (sand)wishes: which summoned a tasty sandwich
Potions of invisibility: either it made your clothes invisible or you invisible, but not your armor
Ring of breathing: which gasped for air when you wore it
Floating disk: that was 5ft from the party, always. Thus no one could touch it
Sword of never miss: it only delt 1 pt of damage.
Sword of ultimate damage: it dealt massive damage, to the air, the scabbard, itself