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The client did not crash, however on quitting/unloading a multiplayer world, a large array of errors popped up. Much of the stack trace involved this mod.

Seeing as this isn't a crash and more just unhandled exceptions I don't think it's a big issue, but it should be documented none the less.

That's unfortunately a vanilla issue which has been fixed in 1.13 (or 1.14, don't remember).
Minecraft doesn't clear the queue of yet-to-be handled packets on disconnect, so those packets are still handled even though the world is already unloaded. Having packets for multiple worlds just makes it more noticeable.
I could clear the list manually but some mods actually rely on that behavior (because they use it as a general task queue), so I'd rather not do that.