Overfilling it could be a problem, I suppose. The real culprit, for me, was always how restless a sleeper I am. The more I moved around at night, the more likely I was to find myself off the mat. This seems to happen more when I'm really sore from hiking: legs ache and become restless, or I find it hard to get into a comfortable long-term position and switch side frequently. (A slightly soft pad, not quite inflated to drum-tightness, helps with comfort.) More often than not, though, I don't roll around very much and don't come off the pad if it's level.