as i understand it (probably need more testing) here is how it work:
when you disconnect when riding an entity, the entity is disconnected and stored with you
if the entity (like the pig) is riding other entities (like the cart) it disconnect all the stacked entities but only store with you what you are directly riding
so if sethbling tested disconnecting on his mob tower with a boat on top, he would reappear with only the boat
also: someone should try a mod allowing to ride other players and disconnect on top of someone to see what it does
as i understand it (probably need more testing) here is how it work:
when you disconnect when riding an entity, the entity is disconnected and stored with you
if the entity (like the pig) is riding other entities (like the cart) it disconnect all the stacked entities but only store with you what you are directly riding
so if sethbling tested disconnecting on his mob tower with a boat on top, he would reappear with only the boat
also: someone should try a mod allowing to ride other players and disconnect on top of someone to see what it does