How to submit hallucinations

[EricAndresen]

I have some cases of in-the-wild hallucinations that are structured like

Prompt: "list (description) products. Only include products that exist".
Response: Detailed explanations of products that do not exist.

1. Is this a valuable eval?
2. How would I structure such an eval? I'm not sure what the ideal answer. "I don't know of anything" or "I don't know of anything like this, but here are some related things" both seem reasonable, but not well structured for an eval.