What subjects current students in MFE wish they knew before they joined program


You see a lot of questions on here asking if they have proper background to get accepted into MFE, but what classes do current MFE students wish they knew better. This could be a class they took in undergrad but did not know it as well as needed coming into an MFE program, or a course they should have taken in undergrad to be more prepared for a current MFE course.