Sometimes you need to prepare a plug to connect different battery packs to a certain connection plug. If you use an intermediate "Easy plug" with just 2 connections you may change the polarity. To avoid this risk, I prepare them using a 3 pins header / socket in this way:
If you connect the 2 sides correctly, everything works fine.
But if you connect them in the wrong way, nothing happens! Just the 0Volts are connected.
Furthermore, you can easily recognize what's the right way.