The Grim Adventures of Billy and Mandy (2001)

S3E3 Dumb Luck

While dining out, Billy accidentally knocks over a salt shaker and neglects to toss some over his left shoulder, triggering a streak of bizarre misfortune. The following day, Grim investigates and discovers that a "Catastrophe Snail" is latched onto Billy's brain, which is the source of his relentless bad luck.