This is what I believe what will happen if you check-in online and your carry-on happens to weight 12 kg instead of 10 kg at the boarding gate:
If you are very lucky, they will let you onboard.
If you are lucky, they will re-issue you with a new boarding pass (that would be 20 EUR) and charge for checking the bag (that would be 7 EUR) and let you on.
In most cases, I guess that you would be denied boarding, all money paid will be lost and if you want to fly out on the next service, you would need to pay a new fare that certainly won't be very cheap as you book on the day of departure. That is what the T&Cs say. You can only hope that you can throw away 2 kgs of your carry-on in that case.
When they announced that they would increase the checked baggage allowance to 20 kg, I was already wondering where they will take all the money that they loose from not charing every kg of excess above 15 kg. Now I finally found out where this money will come from: cases like that one, some BP reissues and the per-bag charge.