The dc power jack on a pedal is switched such that when you plug in a power supply, it disconnects the battery from ground, so that you are not running it on both battery and the adapter at the same time. While modding my tube screamer, I ended up shorting those connections with a jumper so the battery is always connected to ground, because I didn't like the crappy jacks I found with the switch, and the only good quality ones were two conductor. I didn't like the contact connection that closed the circuit inside the jacks, and so I figured I'd see if it affected the tone to jump the connections that would normally just make weak contact. All I have to do is remember not to plug in an ac adapter with a battery inside, right?

But what if I did? What would happen? Would I fry some part of the circuit? What could or would be damaged? They say tube screamers can be run on up to 16v, but I don't think the battery and power supply would be in series anyway, so I don't think it would exceed the voltage limit...