It wasn't hte charge that killed your wireing. Voltage doesn't melt wires, current does. So the fact that the wall wart charger is at 6v or whatever, but only 50ma, it's not enough to melt much of anything.

However, I've had wires go in to melt down mode on nearly drained batteries. 4.4v and a short is plenty.

It doesn't take much, just something that draws too much current. A short is the most common cause, something like a hole in the insilation will do it. I had one battery let the smoke out after a crash, when a chunk of ply nicked a wire lead. The short then melted wires and actually melted a hole in the bottom of the coro and foam fuse.

If you use a switch with too thin of wire (like a micro switch), and then do something like stall a bunch of full size servos (esp digital servos), you might even be able to cause a failure in the switch just from the servos drawing enough juice. I haven't tried this, so I don't know for sure, but in theory it's possible. In the real world, I don't think it's likely.

I did have one servo that had a short inside the servo case, it melted a battery connector on me as well. The servo was fixed by the manufactuer, the battery was toast. Fortunatly, the RX wasn't harmed, even though all the current was going through it's leads.