My postgraduate level classes are somewhat eye-opening. I'm doing quantum chemistry for chemists, which makes it really painfully obvious that I don't know the first bit about Fermi-Dirac (or Bose-Einstein) statistics, and quantum chemistry for physicists, which I'm somehow acing so far.
Extra maths and preparing for this particle physics stuff next semester is interesting, too.
And now I'm teaching a physicist Python because I'm too lazy to write my dissertation in C and have enough freedom not to.
(Honestly, though, does anyone even care about your undergrad once you've got your first Nature/Science/Cell paper? )