In the section on combat, the PHB p191 says "Whether a creature is a friend or an enemy, you can’t willingly end your move in its space."
Even in combat, are there specific rules that override this general rule? For instance, while grappling or restraining a creature. Or hiding behind a creature. Or if you just run out of movement and there you are.
The DMG mentions being in a creatures space in the section titled "Climb onto a bigger creature"
I seem to recall somewhere else where it talks about falling prone in another creatures space, but I cannot be sure.
Your quoted passage from the PHB actually says it all, you cannot choose to do it even incidentally. So this means you can't incidentally run out of movement in a creature's space.
While grappled, another creature is moving you against your will so you can end your turn inside another creature's space if the grappler decides to do it.
As a general rule for RPGs, though:
Players will come up with the most insa- creative ideas and you have to be prepared to rule on what makes sense to you.
Remember the GM governs the rules, not the other way around.
A creature can fly above another, end its turn and lose flying. Similarly a monk can run along a wall, get one an opponent and end his turn thereby falling.
I think falling by however they got above the other character or creature would be the only way.
