I port forwarded my minecraft server and I started building a server lobby, because it was going to be a 'development updates' server. After I built the lobby, i made a single player world so i could beta-test one of my mc 1.9 minigames. I loaded it onto my server (closing the server, dragging + dropping files into the world directory) so my friends could help me test it. So we were testing then my friend disappeared and when i TPed to him, for me, it showed he was at the lobby of the old world!