Two of my singleplayer worlds have crashed because of the exact same reason;
the game tries to load a slime entity and crashes the game until entity is deleted.
I ignored it first time, thinking it was an exception to that world, but now have had the exact same issue on two different worlds.
I use forge and worldedit
Thank you for your report!
However, this issue is Invalid.
Your game, launcher or server is modified.
If you can reproduce the issue in a vanilla environment, please create a new ticket.
Any non-standard client/server/launcher build needs to be taken up with the appropriate team, not Mojang.
Any plugin issues need to be addressed to the creator of the plugin or resource pack.
If you have problems on large servers, such as The Hive and Hypixel, please contact them first as they run modified server software.
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