Can a plug be plugged in upside down?

This question may be limited to Europeans. We seem to have different plugs.

If I understand correctly, a plug has one “plus” and one “minus” pin, both of which are required for the electric current to run through it. That implies the right pin must go into the right hole in the socket, since the “plus” current would be unable to transfer into the “minus” pin and vice versa.
But when I plug in an appliance with the plug upside down, so each pin is in the other hole, it still seems to work, as far as I can tell.
What’s up with that?

(I have a hypothesis, but I’d rather hear the experts first. :) )