I've never known it to cause a problem.
There is some degree of intelligent handshaking built-in.
The powering device (switch or injector) checks to see whether there is a POE-ready device on the end of the line, and only then supplies power.
BTW - check that the powering device (switch or injector) and the powered device (camera, phone, etc) are both to the IEEE802.3af standard (802.3at for high-power / POE+). There are a few oddballs about; early pre-standard devices, some Cisco, some Ubiqiti ...