This is just something I've been puzzling over. I don't have any religious faith now, but I grew up RC, went to Catholic schools and was pretty well indoctrinated in that religion.
So, in the RC sacrament of Holy Communion, the belief is that the host and wine are turned into the body and blood of Christ - the doctrine of transubstantiation. In Anglican tradition, communion is simply a commemoration of the Last Supper and of Jesus giving his life, his human body and blood, to redeem mankind. This is how I understand it, others might see it differently.
So (let's just say that the RC doctrine is "true"): if a Roman Catholic takes communion at a Catholic Mass, he or she is actually taking in Christ's flesh and blood - what about if they take communion at an Anglican service? The words said by the priest/celebrant are the same. Is the act of transubstantiation performed by the celebrant, so RC priests can do it but Anglicans can't? Or, if Jesus created the ritual with his own words, does any ordained minister repeating those words make it happen? Or is it in the belief of the communicant - so if you believe it, it is, and if you don't, it isn't? Suppose a Catholic takes communion in an Anglican church believing it to be a Catholic church (this happened to Italian parents of someone I know) - have they not had the "real thing"?
Sorry this is quite long!