Ok i was playing skyrim on my xbox 360 i was near the edge of the map near the city of dawnstar, when i climbed up this hill my charcter fell through, so i restart my game and do it again and nothing happens,

the next time i fell through the map was when i was in sarthal, i was at the spot were you fight the undead guy, and i fell through the map again, i saved before i fought him, but it keeps happening?

is this normal?