My company uses BitBucket Server for our git repositories. In the UI, if you click the "approve" button on a PR that's already been merged, the message you get is "Participants may not update their status on merged pull requests."
The fact that it's already merged is not the first thing the user sees, and is not highlighted, and therefore easy to miss because the verbiage used is passive. It would be much clearer if it used more active and clear language directed at the user, such as "You cannot approve this pull request because it has already been merged."
