Wei Guan
Wei Guan was one of the few officials who dared to openly speak to Emperor Wu about his choice of heir apparent, his son Sima Zhong, who was developmentally disabled. On one occasion, Emperor Wu, after Wei Guan hinted that Sima Zhong should not be crown prince, sent a number of inquiries to Sima Zhong to have answered. When the inquiries were appropriately answered (because Sima Zhong's wife Jia Nanfeng had someone else answer the inquiries for Sima Zhong), Emperor Wu was happy and publicly showed Wei Guan the answers, embarrassing Wei Guan greatly and making it clear to other officials that Wei Guan had said something.