I have also seen some of the cheap knock-offs (i.e. virtually all of the ones sold on eBay) that can be reversed without a whole lot of effort. I've also seen knock-offs with reversed wires, which is even worse, and with very loose connectors that can pop out or loosen when connected. I get a black Sharpie and mark the sides of black ("-") wires on both sides of the connectors... that way you just have to match the black stripes. And, push the wires inwards to make sure they're 100% seated, and tape the heck out of them so they can't come loose.