I had to break the (bad) news (gently) several times to people who brought mystery shells to the museum where I worked. They had been unable to find the prize in any local shell guides. Some had concluded they must have a species new to science. Some of those SW Pacific shells were even acid polished with trimmed lips! 

The occasional visitor not liking my conclusion, replied they would find a "professional" who did know shells to correctly identify it. 

Walter Sage once related to me identifying an Aliger gigas a visitor to AMNH New York brough in for identification. They had found it on Coney Island, New York. He told them he did not doubt they had found it but that the shell had not lived there. The disbelieving visitor wondered how another person could throw away such a gorgeous shell to begin with as the most logical explanation for how it came to be there.  

I had a coworker with a friend with a big shell she found on a North Carolina beach but was never able to find it in any guide books. She encouraged her to come to the shell show as there were certainly people who would be able to identify it. Her friend's husband scoffed at the idea saying the shell show people at the show would not give his wife the time of day. The shell was in fact a Cassis cornuta. When they spent a couple weeks on holiday the the wife got up crack of dawn every morning and when out the the beach to find a prize shell of her dreams. By the end of the trip, she had been unsuccessful. Husband went to a local shell shop and asked the proprietor to sell him a spectacular big shell that would be found locally. He told the owner his plan so it had to be a local shell. On the last day he got up before his wife, went to the the beach, "planted" the shell, then sat on a nearby dune to be sure the shell would not be accidentally disturbed by another beachcomber. When his wife came out, she inquired as to why he was already on the beach and he said he wanted to see the sunrise on the last day of the trip. Wife soon found the shell and was thrilled. Husband had kept the secret for all those years. He had subsequently discovered the shell did not live in North Carolina thus why he strongly discouraged wife from getting the experts to look at it. Everyone who heard the story thought it was very sweat. Wife was angry.   

John