In my room, in Fort Dawnguard, the mannequin sticks out from his usual position, and is standing in the middle of the floor, but i still have to select the invisible spot where he should be to change his armour...

EDIT: Nevermind i went out the Fort and back in, and it was back to normal

EDIT... again: However the weapon rack is screwed up..

Does he know the two in riften?

Probably not. Ghost mannequins are fricken scary in Riften. I'll run down the stairs and BAM IN YOUR FACE

I'm guessing this is a common problem :')

I dont stay in my owned houses long enough to notice xD

Yeah, it is.
But it seems it only happens in Riften, for some strange reason :/

But yeah, leaving and entering again usually fixes it.