Short story before:
I need to monitor count different http response codes for url provided by my application.
For this case I want to create lld rule which will create following items prototypes:

 - url.counter.response[{#GOOD_CODE}]
 - url.counter.response[{#BAD_CODE}] + trigger prototype

So my lld rule reply looks like:

 {"data":[
{"{#GOOD_CODE}": "200", "{#BAD_CODE}": "403"},
{"{#BAD_CODE}": "404"}]}

But since second json object doesn't have {#GOOD_CODE}" macro, zabbix show error:

Cannot create item: item with the same key "url.counter.response[{#GOOD_CODE}]" already exists.

I found this error message very misleading and think that it shouldn't be shown because:

items like key_name[{#MACRO}] are created by default in lld rule, and items (in my case) url.counter.response[200], url.counter.response[403], url.counter.response[404] were successfully created too