Noted for being outspoken by Craven, Du Plessis was soon in trouble with South Africa's rugby administration for wearing denim trousers with his Springbok blazer. His liberal views, in which he was influenced by Frederik van Zyl Slabbert, and his subsequent support for the Progressive Federal Party, also ran afoul of the conservative politics of rugby administrators. Nor was Du Plessis a stranger to controversial incidents during his domestic and international career. In the 1975 Springbok trials prior to the French tour, Du Plessis punched Kleintjie Grobler, knocking out the man that a journalist later described as "pesky" and an "enforcer". Du Plessis and Grobler then played together on the national team against the Tricoleres that year. But probably the most notable incident involved Naas Botha, whom Du Plessis injured with a bone-crunching late tackle in the 1977 Currie Cup semifinal between Province and Northern Transvaal. The subsequent penalty kick by full-back Pierre Edwards was successful, and Province lost the match 17–15. The Loftus Versfeld faithful was so incensed that Du Plessis had to be escorted off the field by the police.