What if I lose the Carnet or it isn't complete
If you can't get your Carnet stamped by customs
There are occaisionally times when you simply cannot get the carnet stamped in time for your flight and missing the flight may not be a comfortable option. If this happens, then in theory, the country you left does not have a record of you exporting the items and you may be liable for import duty.
Tell your Carnet issuer as son as possible
Once all the vouchers have been returned and married up, one will be missing and the carnet issuing authority will wait for the foreign country to makes its claim. It then is a question of proving that you did not leave the items in that country and, providing you successfully re-imported the items into your home country and have a stamped voucher to prove it, the carnet issuer can present this to the foreign country as evidence and that may well suffice.
If you've lost your Carnet completely
If you have completely lost the carnet, and a claim is made, then you can arrange for customs officials to visit your premises and do a "physical check" and provide a "certificate of location" - basically look at the items and serial numbers to prove you brought them home. This should avoid having to pay the full amount of duty but there may well be an administration charge.
If anything like this happens, contact your carnet issuer for advice as soon as possible.