Just an expression. As speedy_r6 said, things can go very wrong. My experience with batteries is no matter the use, I go for authentic. In the case of electronics, a bad battery could result in damage to the circuitry. Even for a flashlight I want to be sure they're authentic. I could buy cheap, knock off Duracells, but they will likely not last nearly as long as an authentic, and given that if I need a flashlight, I really want to know it will work. Standing in the dark doesn't cut it for me.
In the case of vaping, as near as I can tell, bad things always involve the battery. Whether it's misusing a battery in a mechanical mod, to a cheap knock off behaving badly, the idea of either a smoking battery igniting the desk to a catastrophic failure with an explosion while in use just doesn't seem worth the few bucks to cheap out on. If you buy a poorly produced clone tank, pretty much the worst you'll have is a bad vaping experience. If you buy a poorly produced clone battery, you could end up with bad medical expenses.