Id's Quake and Doom sources are a classic in this matter. I looked a bit at wsw, a fork of Quake 2. Now, my code is full of structs with function pointers. It's like I just internalized the more elaborate syntax and now it's all Hammers and Nails. I mean, function pointers to functions returning function pointers, for example.

  typedef int (*foo) ();
  foo * bar ();
  foo * p = bar;

not rocket science, but ambiguous at first sight when compressed to one line. I hope that kind of syntax is as complicated as it get's, because I'd like to think C semantics aren't really difficult. That is until implementing customized data types on top.

As mentioned in another comment, learning the toolchain is just as important and it's not without reason, that Don Knut takes forever writing about compiler design. Never mind the ventures into the algorithmic side of things.