valac generates interface virtual function wrappers like the following:

properties
----------
gint interface_get_foo (FolksExtendedInfo* self) {
  return INTERFACE_GET_INTERFACE (self)->get_foo (self);
}

void interface_set_foo (FolksExtendedInfo* self, gint value) {
  INTERFACE_GET_INTERFACE (self)->set_foo (self, value);
}

methods
-------
gint interface_bar (Interface* self) {
  return INTERFACE_GET_INTERFACE (self)->bar (self);
}

If 'foo' is a property or bar() is a method added to the interface after an implementing class implements the interface, accessing the property or calling the method on the old class will cause a segfault and otherwise put the client program in a very bad state.

This situation can easily happen if a library exposes an interface which is implemented by a third-party library (which won't necessarily be up-to-date with the latest interface version), breaking backward compatibility in a very fragile way.