It's been the primary and most annoying interop issue for us when we have to integrate C++ code into our primarily C codebase (= build some pieces of our code in C++, which necessarily has to interact with at least our header files).

(Second place: differences in available GCC/clang compiler extensions between C and C++ [our software does not support Windows/MSVC], third place: differences in what casts are permitted/how they are done)