Another really common one is that casting from void * to any other type doesn't require a cast in C, but it does in C++:

  #include <stdlib.h>

  int main()
  {
    int *foo = malloc(sizeof(int));
    return 0;
  }

That works in C, but not in C++.

There's actually another subtle different in there that main() means "unspecified arguments" in C, and "no arguments" in C++. ("No arguments" in C would be main(void).) However, it's no longer commonly used that way in C, but casts from void * to other types is very common in C.