When attempting to use, as a callback, a method which implements an interface, the generated code will attempt to reference what the method name would have been, had it not been implementing an interface. Example follows:

public interface Foo {
  public abstract bool poll();
}

// Works if you remove Foo.
class Bar : GLib.Object, Foo {
  public bool poll() {
    GLib.stdout.printf(".");
    return true;
  }

  construct {
    GLib.Timeout.add(1000, this.poll);
  }

  public static int main(string[] args) {
    var foobar = new Bar();

    return 0;
  }
}

/*
nemequ@desktop:~$ valac -o hurumpf --pkg glib-2.0 hurumpf.vala 
/tmp/cc4YFOCJ.o: In function `_bar_poll_gsource_func':
hurumpf.c:(.text+0x159): undefined reference to `bar_poll'
collect2: ld returned 1 exit status
error: cc exited with status 256
Compilation failed: 1 error(s), 0 warning(s)
*/