Nathan’s Coney Island- whiting in lobster roll
One Red Lobster location- langostino in lobster bisque
Soupman of Seinfeld fame- lobster in lobster bisque
On a related note a few years back there were restaurants selling grouper but substituting it with something else (can’t remember what). It may still happen in some places, who knows.
When I order lobster it would be hard to fake it–I always get it as a steamed whole lobster. 
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Fishes are intentionally/ inadvertently substituted often. Sometimes its hard to tell the fishes apart. Sometimes restaurants/ supplies are just cheating.
