Further Maths, my school didn't do it so I could've gotten away with not doing it and focusing my time on achieving good grades in my other A-levels...but I insisted on doing it anyway as a self-taught A-level (with some extra help from school), which meant me doing my Maths A-level in 1 year and that took my attention away from my other A-levels causing me to slip low enough to miss my firm and insurance offers. It cost me big! Not to mention I failed miserably in Further Maths so I didn't even get something good out of it!
That said, the little bit that I'd learned from it did help make first year uni maths go much more smoothly as lots of familiar topics popped up.