I used to think that UPS were golden. I've recently sent a parcel (signed for) to someone and it did not turn up. UPS were able to produce Proof of Delivery, but she insisted it was not her signature and had been in all day. I suspected that she was lying initially, but reasoning it through decided that I really had no reason to do so.
When I kicked off at UPS the parcel magically arrived with her, so the POD was clearly faked .