Although it's true that making a Horcrux seems very intentional, perhaps spells on the vessel ahead of time to prepare it or a spell to send the soul piece into the vessel, it isn't always. Look at Harry. V's soul latched onto him anyway, without any prior setup or incantations.
Now it appears true that this doesn't happen every time, otherwise every Death Eater would have at least one Horcrux, if not many. (Wouldn't that be a horrid thought -- plot bunny, anyone?) But I think under certain circumstances, it might be possible to have unintentional Horcruxes.