Just yesterday, I was helping a friend with his Futaba Receiver. Not sure the model, but it was a 7 channel full-size receiver. I plugged a battery directly into the battery slot without regard for polarity and smoke poured out from the wires to the battery. I do this all the time with my JR receivers and no trouble. The red wire is on the center pin, so plugging it in backwards puts the "hot" (red) wire in the same place but the ground (brown/black) ends up where the signal should be. This should cause no problem.

But it certainly did on this particular Futaba receiver. The receiver was not totally damaged becasue the pins inside the battery slot melted off. we just used the channel 7 slot for the battery from here on out.