Seeing the stamp is part of the novelty in my opinion. I'd be a little peeved.
It does happen and they won't normally do anything about it.
The stamp is really meaningless once it's canceled and that's what they do when they put it on your form. They write the serial number of the firearm on it and that is considered cancellation.
You didn't pay $200 for a stamp, you are not buying a stamp, waiting for a stamp, needing a stamp or any of that other silliness. You are paying a tax on the transfer or manufacture of an NFA firearm and the stamp is just proof that the tax has been paid. The form itself if approved is also proof of the tax being paid. So as long as you have the form with the approval and signature at the bottom you're fine without a stamp. Been doing this a long time and had a lot of experience with missing stamps, it has never been a problem with ATF on any compliance inspection or other situations.
Frank
While that is true, that's not really the point(at least not to me). Part of the "experience" is getting the stamp. I have a stamp collector t-shirt that I got from DBTT. It's like keeping the dealer window sticker when you buy a new car. There's no real reason to keep it, but a lot of people like to have them.