Just that really.
We have a bedside cot and I put ds (7.5 months) down in his cot bit at night, but now he can roll he invariably ends up in the middle of our bed.
Lady night I went up and we was right by the far side of the bed, another roll and he would have been on the floor! 
I am bfing and he still wakes every 1.5 to 2 hours every night, so I am loathe to put him in his cot in own room. I am already knackered and don't want to get even less sleep!!
Is what do people who co-sleep do when you get to this stage? Not sure that mattress on the floor is an option.