Could someone explain to me why adding a virtual function to the end of a class declaration avoids binary incompatibility?

If I have:

class A
{ 
  public:
    virtual ~A();
    virtual void someFuncA() = 0;
    virtual void someFuncB() = 0;
    virtual void other1() = 0;
  private:
    int someVal;
};

And later modify this function to:

class A
{ 
  public:
    virtual ~A();
    virtual void someFuncA();
    virtual void someFuncB();
    virtual void someFuncC();
    virtual void other1() = 0;
  private:
    int someVal;
};

I get a coredump from another .so compiled against the previous declaration. But if I put someFuncC() at the end of the class declaration (after "int someVal"), I don't see coredump anymore. Could someone tell me why this is? And does this trick always work?

PS. compiler is gcc, does this work with other compilers?