A reporter from the radio show This American Life heard a story he couldn't believe was true - that hog rectum was sometimes used as imitation calamari and served to unsuspecting diners. So he looked into it himself.
Here's the report. Despite a source in the story reporting first-hand knowledge of the alleged switcheroo, there's some debate over whether hog parts could pass as squid. So staff from the show went to the market, fried some up and served it. Most of the people who tried it couldn't tell the difference between the hog parts and squid. via This American Life