ATTI mode will not "cause" it to take off or crash into a wall.
The difference between ATTI and GPS mode is GPS. GPS mode will hold it in place even in wind. Remove the GPS and now you have ATTI mode. It will hold its height but will drift with the wind. You are in control of its position.
ATTI mode dosen't cause anything but if you don't keep it in place, it can drift into a wall....