Galois theory is pretty awe-inspiring in general. Loosely, the ‘fundamental theorem’ of Galois theory provides a one-to-one correspondence between field extensions and their automorphism groups, subject to some conditions. This allows you to study field theory with group theory and vice-versa, which can be quite powerful. It takes a second course in abstract algebra to fully appreciate the theoretical depth of this subject, and it is even more amazing because Galois invented it when he was 17!

Complex analysis is also full of surprising and unexpected results. My personal favourite is that the integral of a holomorphic function is invariant over homotopic curves. For some reason which I can’t fully articulate, I found this very bizarre and unintuitive.