Ethernet device voltage protection
The magnetics will protect the PC, network card, chips, everything behind them etc., but they won't necessarily protect themselves. If you provide + and - on the same pair then high-current will flow through the transformer on the other end (potentially damaging it).

You can decrease this risk by:


Add polymer fuses on your lines (if that is possible).
Make sure, that you don't provide voltage difference to a single wire pair (and assume that user's Ethernet wiring adheres to standards). Eg. provide +5V on pair 1 and GND on pair 2.