The Grim Adventures of Billy and Mandy (2001)

S7E5 Billy Gets an "A"

After Billy flunks a crucial test that will affect his permanent record, he enlists Grim's help to magically boost his score from an "F" to an "A." Reluctantly, Grim grants the wish, turning Billy into a brilliant scholar, but this shift disrupts the fabric of reality. Mandy and Grim attempt to reverse the change by traveling back in time to prevent the younger Grim from assisting Billy, but their efforts repeatedly fail. They venture through various historical periods, even trying to stop Billy's ancestors from meeting. The episode concludes unexpectedly when they encounter Billy's prehistoric relative, leaving Grim pondering the consequences.