MH Bulb in HPS Ballast?

What is wrong with doing this? Don’t just say your not supposed to cuz it’s a fire hazard.
Ive heard people arguing both sides of this. Some say they do it with no problem. Some say it’s a fire hazard. The Sun System 4 accepts both and theres no fire hazard, but I don’t think its designed any differently than the original sun system, which I have(150 hps). I don’t see why I cant just stick an mh bulb in there.
Whats the deal with this? Can you safely operate a regular Mh bulb in a HPS ballast?
Also if you do this do you need to change wattage(ie. 175mh for 150 hps) or do you use the same wattage bulb?