I said public domain licensed documents may not be released, not public records, though it's true there are cases where public records can't be released.

So for example, if the government hosts an event and makes posters for it, where the whole poster is a piece of stock imagery that they've licensed for the event with a little text on top, you can file a public records request for "all promotional materials related to X" event, and you'll get a photo scan of that poster.

...but the stock photo is not public domain, thus you can't just copy that poster and use it yourself, because you don't own the rights.

Public record, not public domain. The license of the work or its components is separate from the legal classification of that work as a public record or not.

However, yes, there are also legitimate examples when public records can't be released.

For example, you have the right to request things like body cam footage from local police departments. But if that department is doing a long-term undercover operation, they can't be releasing all the bodycam footage until that operation is over, otherwise they'd reveal the identity of the undercover officer.

You can also make a public records request for essentially anything, though. So there are instances where the request is simply unreasonable and will never get fulfilled, because people are simply making public record requests for things that aren't considered public records.

If you wanted to request some documents about how the military is purchasing certain types of goods, you can probably get that, but you'll find it hard to get the exact GPS coordinates of every military base and nuclear silo we have, because that would make them incredibly easy to target by an adversary.

You can request certain correspondences by a government employee if they're deemed relevant to your request and not dangerous to release, but requesting a full copy of every employee at the DOJ's email inboxes would probably not be a reasonable request given data sensitivity, for example.