Reproduction

• Join a dedicated server as an op
• Obtain a Jigsaw and right click to open its UI
• Click "Done"
• Client is disconnected

Notes

Does not affect single player worlds. Only affects dedicated servers.

Client error message:

Potential Cause

When a client clicks the "Done" button on a jigsaw block GUI, h is called, which calls j, which attempts to send an om packet to the server (a jigsaw block update packet).

However, the om class is not present at all in the dedicated server.