If you have a const delegate that refers to a private method this reference will show up in the .vapi.

For example:
---
public delegate void Callback ();
public class Foo
{
    public const Callback cb = private_function;
    private void private_function ()
    {
    }
}
---

$ valac --fast-vapi=foo.vapi foo.vala

Will generate:
---
/* foo.vapi generated by valac 0.24.0, do not modify. */

using GLib;

public class Foo {
	public const Callback cb = private_function;
	public Foo ();
}
public delegate void Callback ();
---

Which doesn't work because 'private_function' is not defined.