Note: First and foremost it will not power up because the power-on pin (#14, most often green cable) is not connected to ground (any black cable).

https://www.google.de/search?q=atx+power+supply+pinout&c...

The most likely outcome though is that the supply will just power on and idle without any load. Rapid transients on the rails (say from 100W to ~0W) might cause the over voltage crowbar to trip, though.