Quantum Field Theory
Quantum Field Theory has the reputation of being one of those subjects that separates the men from the boys. The boys, of course, go on to become particle physicists, solid state theorists, and so on, while the men find less challenging careers. I'm not sure if it's still possible to get a PhD in physics without taking a QFT course but I am quite sure that it's possible to get one without understanding it. I did take, and pass, such a course once long ago, and I've since worked a hundred or two quantum field theory problems, but I can't pretend to understand it in any deep way.
It's possible that others share my intellectual handicap. Maybe that's why quantum field theory texts continue to bloom like leaves in the spring. It's hard for me to believe that there are as many theoretical physicists as there are QFT texts, but I suppose that I must be mistaken. Anyway, I'm pretty sure that I own less than 100 of them, though it has been a while since I counted. Unfortunately, I do keep buying more.
I tend to blame Amazon's book preview feature. From time to time I see a new title, browse, and see something that seems to explain a point that was always murky to me. A few days later, another damn QFT book arrives in my mailbox - or on my doorstep.
Another problem I have is that I like to start books from the beginning. That usually results in my falling asleep sometime between the first mention of Noether's theorem and renormalization.