Unless you encounter a reverse polarity socket!
I must admit that was my first thought too, but then I think the chargers in a lot of vehicles only make use of L1, some can make use of all three, but as you say... what the control logic would make of being presented with one phase missing and the other two the same phase is anyones guess.
I'd like to think it checks that both are plugged in before connecting them together, but even if it does, if one plug is pulled out can it detect that and disconnect it, I'm sure its possible, but its a lot harder than just doing the first bit which could be done with a couple of relays
I've heard that cars that can utilise a 3ph charge point just have 3 single phase chargers on board,,, and not a single 3 phase charger