I've seen this mentioned in threads but not really detailed or discussed. There may be additional effects or events then the three I listed, but they should operate on the same core principal.
The concept of a character being put under the control of another player,being or entity shouldn't be taken lightly. Like the imprisonment debate, no one wants to lose control of their character. But sometimes the risk is considered worth the possibility.
A prime example is Michael Moorecock's Stormbringer, the black sword possessed by a demon that grants great power and extracts an equally great price from its holder. In the books the sword would, at times, move of its own violation and suck the soul of someone standing nearby. Often the victim would be someone Elric considered a friend, it seemed to favor slaying any female Elric was involved with. But the power granted by the sword was awesome.
An example in a game is the Barbarian class's Berserk ability in the 2D graphical MUD Drakkar. In that game a Barb could activate his Berserk ability while in combat, the ability had levels which were accessed with repeated key presses of the ability button. As the Barb increased their "rage" their damage, number of attacks in a round, ability to dodge would all fluctuate randomly within a round while the Barb would often let out wild, incoherent screams. But there was also the chance that the Barbarian would lose control, leap huge distances and strike (usually with massive damage) friendlies, party members pretty much anyone. The way a Barb usually "calmed down" was to go off where they alone until they naturally calmed down, think of it like some games handle recovering from having drank too much.
Demon possession would be putting a character under the control of an NPC demonic (or Devilish?) entity.
Cursed items, perhaps even cursed locations might result in a character becoming possessed.
Mind control would be putting a character under the control of another player, or NPC entity.
Now my thought would be to have these things be pretty much short term events. No one wants to spend 30 mins watching someone else kill your alignment after all! But I do think this sort of thing would provide a lot of RP potential as well as being something memorable to all parties! :D methods to rid characters of these effects shouldn't be too difficult to find nor perform unless its some sort of special situation.
Spells in the form of Dominate Person, items or the effect of some action by the player might lead to their becoming possessed or some sort of divine interaction may result in a player being possessed (perhaps not all possession need result in bad things happening? That would be more of a GM event though) . In terms of the Barbarian, the skill Berserk. Perhaps there are other things that can cause these effects or even other similar effects.
It goes without saying that this would be something far back on any priority list if it even makes a list!
So, what do you think?