How expectations can turn a friendly submarine into a U-boat

Why might a Coastal Command B-24 crew ignore signs that a submarine was not a U-boat?

Because the crew believed no British submarine should be in the area, so its operating picture made an enemy identification seem more likely.

The answer reduces the incident to a classic fog-of-war problem. The aircrew did not believe a British submarine should be operating in that location. Once a submarine was sighted, the prior assumption made a U-boat identification appear reasonable despite contrary signs. The exchange shows how an incorrect operating picture can outweigh visual clues during a fast contact.