Thankfully, this is utterly illegal in most countries. Of course the biggest difficulty will be to prove that this is what happened.
If it's carbon paper, it's easy, "just" keep the original, and have an expert compare the signatures. If it's a square hole... well, keep the original, but it's tougher.
Marriage CertificateUisin-ah, could you please sign this?HahaOops, my mistake.Here
