When a Discovery Rule has an issue and doesn't return valid JSON, there's no way to be notified of this unless you happen to stumble across it in the GUI or the effects of it (missing items). In both cases, the rule could be malfunctioning for some time without admins noticing.

I had a discovery rule using an external script that suddenly started failing (java version issues. ugh.), but didn't notice until I saw the little ! icon next to it in the host's discovery items list by random chance. The externalscript was erroring, and thus the output wasn't valid JSON.

To some extent, I can make the script cover this by posting back with Zabbix Sender every time it runs... but this seems like something that should be more visible from the Zabbix system itself.

Ideal solution would be to allow triggers for the discovery rule itself. Much like how Web Checks have a variety of items which can be checked, Discovery Items could return error strings, number of items found, number of items pending removal, etc.