That's funny.

I discovered an off-by-one error in college as I was graduating. Apparently the "class plan" I had put together with an advisor during my second year was missing a class, and I discovered it as I petitioned it for graduation. "You're one class short."

(It was for my second major, not the primary one, and the head of the physics department was nice enough to credit a nonlinear optimization course from engineering toward the major, so I earned it.)