showing one card

You're playing hold em, get to the showdown, you show one card, which you think wins. The opponent shows both of his, which beats you. Then you realize that your second (down) card, in conjunction with the first, trumps his claim.

I doubt this has ever happened, but I wonder what the ruling would be?

You get dragged into the street and beaten to a bloody pulp for slowrolling, and learn a valuable lesson to show both your hole cards on showdown when you think you hold the winning hand.

'I doubt this has ever happened, but I wonder what the ruling would be?'

T.J. C. accidentally did this to Phil H. at the final table of some tournement.

T.J. thought he had spade-club and said 'I think you got me' then he turned over the second card, which was also a spade giving him the nut flush.

Phil H. fell out of his chair (literally) because he thought T.J. slow rolled him intentionally.

Don't quote me on the suitedness it may be reversed but you get the point.

Fell wrote 'You get dragged into the street and beaten to a bloody pulp for slowrolling, and learn a valuable lesson to show both your hole cards on showdown when you think you hold the winning hand.'

That's the proper course of action if someone slow rolls you or if you slow roll someone else. Turn both cards over (or muck them) and let the dealer make the ruling.

Your not giving away that much information.