Whilst most of the incompatibilities exist only in theory between the ISO99 standard C and C++ there does exist some problems such as:
extern int foo();
Is valid C but will cause an error in C++, in 2 places with g++ since main must return an int, and there is too many arguements passed in foo, in C it is assumed that extern int foo() calls a function with an unspecified number of arguements where as C++ assumes it is called a function with no arguements ie: extern int foo(void)
So whilst there shouldn't be incompatibilities some do exist due to language evolution.
Btw most of the incompatibilities on the link included are theoretical also with some compilers the incompatibility I have shown will be detected with others it won't according to the standard it is invalid C++ but valid C, and when I tested this will gcc and g++ it showed the problem.