There's a question that Roman Catholics will sometimes discuss, that I have real difficulty getting my head around: what makes a Eucharist "valid."
Behind the question is the conviction that it's possible for one congregation of Christians to pray (either corporately or through their priest), "that these gifts of bread and wine may become for us the Body and Blood of our Lord Jesus Christ," and by the grace of God, this very thing will happen, but at the same time another congregation may offer the same prayer, and Jesus totally "blanks" them.
This is bound up in the issue of what makes a legitimate church. (It won't surprise you that I believe it's possible for a congregation to be completely illegitimate as a church, but I have particular ideas about why that might be.) I do not believe that "legitimacy" is passed down through a mechanical "apostolic succession" down two thousand years of history, such that God can be invoked by some priests and bishops but not others. When I hear an argument that I find convincing, trust me, you'll know.
Here's the thing: remember our little discussion on what sacraments are? The bottom line is that we beg for the healing presence of Jesus that has been promised. I think that the triune God would have a really big and obvious reason for denying our entreaties for the crumbs from his table, and I'm not terribly sure what that would be.
When we start talking about the rules and special circumstances by which God will mediate his presence or not, it seems to me that we move out of the categories of promise and gift, and of begging and humbly receiving, that make Christian sense of the whole thing. Discussions about who has whose ducks all in a row get silly very quickly.
A really funny moment that brought this home for me was when an ECUSA bishop insisted that whatever my church celebrated, it wasn't the Eucharist. I almost laughed out loud. Prelates who live in glass houses...
Update: For some background, here's a longer post on apostolic succession.
The Peregrinator at Selva Oscura has written a thoughtful response here.