Probably the one of the big mistakes done by Japanese in the history was the surprise attack on Pearl harbor on 07 December 1941(local time). Japan was already having a war between China at that time and the Unites States sent the final proposal, which is known as Hull note. This proposal included withdraw from China and Indochina which was not possible for Japan.
Japan rejected the proposal and fully proceeded to war against the United States. Japanese Commander-in-Chief of the Combined Fleet, Admiral Isoroku Yamamoto asked his fellow commanders to plan the attack. By the way, Isoroku means 56. He was born when his father was 56 years old.
1st Air fleet commanded by Vice admiral Chuiichi Nagumo first anchored at Kasatka bay of Iturup, Kuril islands. Kasatka is current Russian name but it was called Hitokappu when Kuril islands were under sovereign of Japan. 1st Air fleet was the first and largest fleet of aircraft carriers.
The biggest mistake itself was not the attack on Pearl harbor but staff of Embassy of Japan in Washington D.C. sent the declaration of war after the attack. It is told that the Embassy had a farewell party of its staff and forgotten about the declaration. The declaration of war was passed by the ambassador to Secretary of State one hour after the attack.