I went to pin a message in a room I own, and was puzzled that I couldn't. Another moderator could though. So I tested on another message, and found this is what happens:
- Star or pin an item, then change my mind
- Decide again to pin or star that item
- Can't do it, message when I try to star is "It is too late to undo this operation", message when I try to pin is "You have already voted, but the voting has been cleared by a moderator"
This can be worked around by deleting and then re-posting the message in some cases. Clearly it doesn't come up much either, and you can always ask another mod to fix it if it does and the delete/re-post option isn't good. It has an interesting abuse mode though...
I found I can also post something, pin it, then unpin it, and the star stays even though it is my own post.
It's a minor issue, we aren't at risk of moderators constantly starring their own chat messages. I don't think :) .