How to make ansible task output line `failed: [server]`?

A failed Ansible task can have two kind of log lines:

failed: [server] ...

or

fatal: [server] ...

However I tried very hard to simulate a situation that can have the failed: one but without any luck:

• call /bin/false
• call exit 1
• use ansible.builtin.fail

All of them output fatal:

Any idea why some task has failed:?

failed: [server] only happen in a really specific case: when an item of a loop produce a failed state, as you can see from this line of the v2_runner_item_on_failed method of the default callback plugin:

msg = "failed: [%s]" % (host_label,)

^{Source: lib/ansible/plugins/callback/default.py}

So, in order to create a failed: [server], you need to be in a loop, for example, the task:

- ansible.builtin.fail:
  loop: [~]

Will produce

failed: [localhost] (item=None) => changed=false 
  ansible_loop_var: item
  item: null
  msg: Failed as requested from task