If you place two End Crystals on the ground when using 1.17 Multiconnect to access other versions both crystals will explode serverside but client side only one of the crystals (usually or only the first from what I can tell) will be removed from the world and the other is not. This ghost crystal left behind can't be attacked and just continues to exist - on some servers (probably depending on version) it'll block placing in that location, on others it'll just stack another crystal on top each time another is placed. (Tested on a 1.16.5 and a 1.12.2 server)