While I haven't done it myself...
I do know of one friend who woke to find her boyfriend happily standing urinating on the radiator on the other side of the room, rather than in the toilet as might be expected.

The result of going out, getting bladdered and a combination of sleepwalking and needing the toilet - if you have experience with sleepwalkers then it won't be quite so surprising to hear - They can be found quite happily wandering about in real life, while their head still firmly believes them in the hold of whatever dream it is conjuring up, so the toilet they seem to believe is in the cupboard in to them the one in the pub, or something much less sensible and more dreamlike.
As with most sleepwalking, ask them in the morning and they will have no knowledge or recollection whatsoever of their night time wanderings.

So I guess it depends on your mental stage, if you never sleepwalk to start with, I doubt you will regularly find yourself pissing in odd places, someone more prone to sleepwalking will find it somewhat less surprising should they go to bed with a full bladder.
(442, Wed 2 Mar 2011, 0:57, Reply)