One of the more-overlooked examples is the mission of the Graf Zeppelin II, sister airship to the Hindenberg, up the UK coast just before the war, trying to learn about the British Chain Home radar defense system. The operators heard the radar signals but thought they were electronic noise from the power grid - because the British used their power grid as a frequency standard for the radar system to synchronize the towers! So the airship flew up the entire British coast, watched the entire time by the Chain Home radars, and then went home and reported, "We saw no evidence of a British defense radar system."
It was one of the biggest electronic intelligence failures in history, and directly contributed to the British victory in the Battle of Britain. The German Luftwaffe went in assuming the British had no extensive radar system, when the exact opposite was true.