You have to get the forward pointer when you register the forward then use that to unregister it. Otherwise, it's trying to unregister a different forward.
You should learn to look in the include files for some extra information:
PHP Code:
/* Registers a forward.
* Returns an id you can pass to unregister_forward
*/
native register_forward(_forwardType,const _function[],_post=0);
/* Unregisters a forward.
* The registerId must be from register_forward, and
* post/forwardtype must match what you registered the forward as.
*/
native unregister_forward(_forwardType, registerId, post=0);
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