A question about OTB rules

I've been thinking about it for a while, this has never happened to me, but this hypothetical situation is not too far fetched.

One way to resign a game in an over the board competition is to spontaneously extend your hand for a handshake. Your opponent shakes it and your resignation is official.

Now imagine this. You have a winning position, you make a move that you believe checkmates your opponent's king and, quite naturally, you extend your hand to seal a victory. Your opponent shakes your hand and then proceeds to write on her scoresheet that she won the game. It turns out that there was no checkmate yet, the check could still be blocked one more time before the inevitable mate. But you had extended your hand and your opponent took it as your resignation. You call the arbiter, what do you think the verdict will be? Are there any specific FIDE rules for this situation?

You should never just shake hands to end a game. You need to be very clear what you are offering. So either say I resign first, then shake, or I offer a draw and then offer the hand to shake.

For checkmate, just wait until the opponent agrees that it's mate or stay checkmate and if you both agree, then shake.

Not an answer to your question but this actually happened. Sometime in the 1950s a GM (a GM no less) found himself in a loosing position, he extended his hand in hand shake and said “you played a wonderful game,” he did not say the manic words “I resign,” his opponent taking this as a resignation thanked him and left the room sure enough the GM quietly made a move and waited until his opponent’s time ran out, the GM declared a win on time. After all he did not use the magic word, and in the days before players were required to sign scoresheets at the end of the game he stood by him win. The arbitrator was called in and because there were witnesses to this, ruled that the GM’s behavior was not within the spirit of the rules. The win was given to the player that actually played a better game. I am not sure that this is the origin of requiring a signed scoresheet but an incident like this could well be.

Which GM was this?

Yes, most people will probably do exactly that. But unfortunately there are a few who want to win by any means possible and will use every dirty trick in the book, so I wonder what happens when you run at such a person.

Many years ago, during a FIDE tournament in Europe, a Croat was playing a Mongol.

Late in the middle-game, the Croat said something (he said "Draw?", but in Serbo-Croatian).

The Mongol said something in reply (he said "Pardon?" but in Mongolian).

The Croat thought he said "OK".

The Croat offered his hand (the Mongol thought he was resigning), and they shook on it.

The Mongol then went off to talk to the Tournament Director and record his win, while the Croat... content with his draw... wandered around the playing hall looking at the other games. It wasn't until he saw the 1-0 go up on the cross-table that he realized something was wrong.

Naturally the Croat protested, and an adjudication committee was convened.

The Mongol said that the Croat had said something incomprehensible and then offered his hand. He MUST have been resigning.

The Croat angrily pointed out that no Slav would ever shake hands with a man who had just defeated him!

The adjudication committee found in favor of the Croat, and the game was re-started.

Crazy or not, offering a handshake is the most common way of resigning. Almost no one ever says "I resign". Most people simply extend their hand, sometimes stopping the clock first. If this is simply a sort of "gentleman's agreement" way of resigning, not something recognized in the rules, then my question is, of course, immaterial.