The FBI was warned in advance about the Pearl Harbor attacks, but chose to do nothing about it!
On the 12th of August, 1941, a double agent named Popov informed the FBI of the impending attack on Pearl Harbor.
Popov had been sent to the United States to establish a new German network and during his time there had uncovered information suggesting an attack on Pearl Harbor was imminent. He chose to share this information with the FBI.
At the time, J. Edgar Hoover was the chief of the FBI. He chose not to report Popov's findings to his superiors in light of the fact that Popov was a double agent.
The FBI had been given notice of Popov's arrival by M16 and Popov himself said that Hoover was suspicious and distrustful of him.
At one stage, Hoover threatened to have Popov arrested when he discovered Popov had taken a woman from New York to Florida. This did little to alleviate Hoover's distrust of Popov. Had Popov been able to gain Hoover's trust and had Hoover chosen to act on Popov's information, it may have slightly altered a piece of history as we know it.