Ieyasu's uncle Mizuno Nobumoto did a lot for his nephew. He helped Ieyasu to return back to Okazaki after the Battle of Okehazama and the Oda/Tokugawa alliance in 1562 (Shokutoku Doumei). He also participated in battles such as Mikata ga hara (1572) and Nagashino (1575).
However, Nobumoto made one grave mistake that cost him his life. There was suspicion that he sold supplies to Akiyama Nobutomo, (who served under the Takeda) who was the lord of Iwamura Castle in Mino. This was no laughing matter. Selling supplies to the enemy cost Nobumoto his life. Nobunaga was upset as well as Ieyasu. On December 27, 1575, Nobumoto was forced to take his own life.
Ieyasu had to be in complete shock that his uncle would be helping the enemy. Nobunaga was not a happy man since it was one of his retainers who was caught in the act.
Nobunaga to Kieta Kanshintachi by Katsuhiro Taniguchi. Pages 192-200.
Nobunaga no Shineitai by Katsuhiro Taniguchi. Page 218.
Tenka no tame!