Assuming you plug it into the same receptacle or one that is on the same phase, nothing happens. Stupid for sure, don’t do that, but it’s already wired that way in the wall, you aren’t short circuiting anything with a cable like that (also assuming you didn’t mix up the neutral/hot wires when you made the cable). USA outlets at least. I don’t know about the rest of the world.
Assuming you plug it into the same receptacle or one that is on the same phase, nothing happens. Stupid for sure, don’t do that, but it’s already wired that way in the wall, you aren’t short circuiting anything with a cable like that (also assuming you didn’t mix up the neutral/hot wires when you made the cable). USA outlets at least. I don’t know about the rest of the world.