Not a flat mate, but when my Mum and Dad took in a friend of mine when we were 15... living nightmare! We got on fine to start with (and get on really well now), but found out quite quickly that we were polar opposites.
We had to share a room, but also my double bed. Mum and Dad had only just bought it for me when she moved in so weren't going to get rid of it. I would work all weekend (3 weekend jobs, plus babysitting!!) to afford nice things, such as expensive make up, which she would use, leave on the floor, take with her when she went out etc. and when asked to replace it, would replace it with the cheapest brand she could find. She broke my GHD's, again that I had worked all summer for!
We we're tight for space in our room, so I suggested we have a sort out of stuff. Which she did. While I was away for a week. And by sort out, I mean, threw anything away that wasn't hers or that she couldn't use! I came back to half a wardrobe missing and 15 years worth of keepsakes and trinkets gone! Some were recovered from the bin!
She would leave glasses on the window sill, to go mouldy and stink. She would bring food up and eat in our room, then put the plates under the bed! Which I only realised when the smell got that bad.
She insisted on having an electric blanket all year round!
She had awful clip in hair extensions, that would moult (sp) all over the floor. So if you came in our room after a shower you would get fake hair stuck to your feet regardless of how hard I hoovered the carpet!
I could go on but I think I need a drink to calm me down!