Struggle of a student

Looking back at my university years, there have been several things that I struggled with or thought I could never push them into my brain. Now, in my everyday job life, some of them I never need (phew), while some I am using daily and these suddenly seem so clear and easy to me.

Did you have a similar experience while studying? What was the formula you could least remember or the piece of theory you had to learn but then turned out useless for what you are doing now?