I'm a developer of a custom server software which has a plugin API allowing plugins to control various things such as sneaking.

Previously, when a player started sneaking, resending their entity metadata without the sneak-flag set would cause the client to unsneak. OK so far.

As of 1.1.0.9 (possible also 1.0.x, the reporter didn't say and I didn't test on 1.0), the client unsneaks for a split-second, and then resends the sneak action to the server. Plugins which cancel the sneak event then end up with a client-server feedback loop.

I distinctly remember this worked just fine ~0.16.x, I'm not sure exactly when it became an issue. A demo video is attached. "cancel sneak!" message is sent to the client when the plugin cancels the sneak-event and resends the entity metadata without the sneak flag.

To clarify: Resending the entity metadata without the sneak flag when the player is sneaking ought to turn off sneaking client-side. Instead the client starts sneaking again and resends the start-sneak action.

References:

• https://github.com/pmmp/PocketMine-MP/issues/950