Some of the discussion in this question got me to thinking about an encounter design that sounded fun but I couldn't figure out an elegant way to pull it off:
In some situation where one or more PCs for some reason has a different perception of the encounter than the rest of the group (e.g. hallucinations, illusions, mind-control, etc.), it seems like it would be great to still have that player actually maintain control of their character, but feed them different observations. For example, telling the whole group that they see an evil wizard about to attack, but the afflicted PC sees a damsel in distress about to be attacked by a horde of orcs (who, in reality are the other members of the party).
One naive way to implement this would be to separate the players into different rooms. Of course, at this point the players know something is up, but maybe not exactly what. This also seems fairly tedious (having to go back and forth and keep up the dialog between rooms, converting between the different observations).
Another way is just to keep everyone at the table and try to push everyone towards roleplaying "properly" -- that seems like it would work well in some groups but not others.
Any other thoughts or suggestions about implementing an encounter where players aren't all necessarily having the same observations?