After completing the Ghost House, Wugga jumps in celebration, causing Mario to die. After loading state, he defeats the Big Boo again... and accidentally loads the state in the middle of celebrating, causing him to fight the boss a third time.
Wugga. Special Stage 1. Finnish cursing. That is all.
The infamous KaizoTrap. Mario does his victory march, and then unless you hit a P-switch immediately before hitting the goal tape, he suddenly falls into Grimy Water, negating the very victory he just achieved.