If you are in a 110-125V country, setting the wrong position may do:
- PSU works but likely at reduced efficiency and reduced margins to cover brownouts
- PSU blows its fuse or possibly more
If you are in a 220-240V country, setting the wrong position will:
- almost certainly blow the fuse
- possibly destroy the PSU
- maybe damage computer components if the PSU rings its outputs faster than it can blow itself up
So the wrong position in a ~120V area is relatively safe while the wrong position in a ~240V area is almost guaranteed to blow something up.