That happens to me often enough when posting in other people's LJs. The message is posted, then disappears almost immediately. Sometimes it reappears all by itself, others not, but since I often post terrible things to people, for those cases I just assume the girls have deleted my comments before their boyfriends see them.

If you want a lot of your comments to get deleted, every time any of your friends post boring pictures of themselves at weddings, parties and picnics, make a habit of ranking all their friends by hotness.