1. The mess.
2. Upturned Plugs.
3. You have to kill everything.
4. You find out your not as funny as you thought.
This one came as a shock to me. Before you move in with a girl, you're hilarious. They laugh at everything you say. But once you move in with them you realise they were just trying to humour you.
5. Your clothes disappear, and not in the good way.
6. You learn things you never thought you would.
Before I moved in with my girlfriend I knew various things. How much beer I could drink before falling down, my favourite sport teams stats. After living with a girl for a bit you learn "girl things". I now know how blend make-up and how to back-comb my hair to give it more volume.