In Multivariable Calculus I earned the lowest grade I have ever received in any class in my entire life both up to that date and since (it's the one that isn't really failing but isn't really passing either). It was the nail in the coffin for my budding engineering career.
I still remember sitting there in the middle of the back row in the most uncomfortable wooden desk/chair combos in the H-shaped technology building. It was horrible. The only thing I learned was that I shouldn't go any further with math. The only other math courses I would take the rest of my college career would be statistics, which I would learn to love, but I no longer had any use for calculus.