Rav Ada bar Ahavah had a child who was born circumcised, and blamed himself for causing the castration of the child by bringing him to numerous Circumcisors on Shabbat, against Rav's Ruling: Shabbat 135a
Rav Nachman found Rav Ada bar Ahavah making a blessing on creating his Tzitzit-Fringes, and Rav Nachman made fun of this, saying, "What is this Tzitzi I hear?": Menachot 42a