Where does the post that 'An Post' loses go?

How does 'An Post' lose post? There are at least two piece of post sent prior to or during the Christmas period that have never arrived. One is a Christmas card from a friend who did send it. The other is a DVD from Screenclick.com.   OK so we know that the run up to Christmas is a busy period. Manic I'd assume for the post office. But I still don't understand how post is 'lost'. Does this mean there is a vortex in the postal sorting center where random items of mail venture only to reappear in the dim distant galaxy where they live with odd socks in perpetuity? Do items of post fall through cracks in the space time continuum where they are altered into wire coat hangers and reappear randomly in wardrobe across the land? Or does someone in the post office say "Ah sod this" and chuck bags of post into the rubbish? I can understand why post takes a bit longer to deliver over Christmas. I can even understand why it may take a lot longer and may go via Mullingar and Terminfeckin. but I don't understand why it gets lost. Where does the post that 'An Post' loses go?