From Scholars Before Researchers: On the Centrality of the Dissertation Literature Review in Research Preparation by Boote and Beile (2005):
We have all heard the joke before—as we move through graduate school, we learn more and more about less and less until we know everything about nothing.
I hadn’t heard the joke before, but I think it’s hilarious. Must be the limits reference.
Victor A. Barger 
I’m in grad school and I’d have to say this is 100% right. I can emphatically say I know so much about nothing right now. It’s actually pretty scary. But yeah…Grad school is a trip! One piece of advice. Make good friends. You’re going to need them.