If a player enters a portal, and a second player enters  the same portal within 10 seconds the second player will be kicked from the server with an "Internal Server Error" (see https://hasteb.in/ciqisofo.apache for error). However, if the second "player" is a non-player entity, for example a zombie pigman then the server will completely crash, it will disable all plugins and hang until the server is forcefully stopped and started back up manually; this also causes rollbacks of the world because the world fails to save.