If you didn't have rights you wouldn't see the board at all. Roving Crits is not a private board, but one available to all members. There are no 'posting' rights, just rights to see (and post and edit) boards. Private Roving Crits is an example of a private board that you need rights to see.
There is no moderation before posts get made. A mod might remove a post if it contravenes TOS, but nothing happens before.
I don't have more information on the technical side, so I'll let those in the know have a look in that area.
There's one other possibility. If you hit preview instead of post (maybe by accident? I've done that), it wouldn't have posted. Glitches in connectivity are certainly possible. As far as I know, they are extremely rare. Other than that, Cat's got you covered for the other info.