Of course you have been trained, but what about the fun factor? I mean people love to fit, insert things into other things and feel they fit. Kids feel good when they insert a cube block inside the square hole.
USB cables and Apple batteries design are very good in this regard. No need to look at symbols. It gives you instant sensitive feedback. I'm highly kinesthetic so that is brilliant design for me and a lot of people. Like blind people.
USB plugs/sockets are awful. Sure, you can't insert them the wrong way round, but visually it's hard to see which way round is the right way without trial and error. The position of the USB symbol is supposed to help, and does to some extent.
USB plugs should either work successfully inserted both ways round (like this battery system), or be asymmetrical (and even with asymmetrical plugs, things can get annoying when you cant get a clear look at a socket - eg SCART + the backs of AV equipment)
USB cables and Apple batteries design are very good in this regard. No need to look at symbols. It gives you instant sensitive feedback. I'm highly kinesthetic so that is brilliant design for me and a lot of people. Like blind people.