Steps to reproduce:

1. Create a template with Discovery rule of type External Check.
2. Assign template to host.
3. Make the external check script returns an invalid JSON
4. Observe the web interface for the host saying this discovery is not supported.

Result:
The discovery rule is never run again. Clicking on "not supported" link shows the rule as "Disabled". The rule in the template is, in fact, still "Enabled". There is no way to re-enable the discovery rule from the web interface aside form removing and re-adding the while template to the host.

Expected:
The discovery rule should run at the specified period no matter whether the previous run was successful or not (just like checks on all actual items do).

Notes:

This may or may not be related to the type of discovery, External check. I don't think I've seen the described misbehaviour before with more usual agent-driven discovery scripts.