As a moderator on a SE site, I recently observed behavior that merited a suspension. I took the time to write a lengthy, detailed private moderator message to the user, to explain the reasons for the suspension and explain how the user could contribute productively when they returned.
However, another moderator beat me to it: they saw the same behavior and suspended the user while I was in the process of writing my message, sending their suspension before I finished writing my message. When I finished writing my message and clicked the 'Notify User' button, I was greeted with the following error message:
A more recent message has been posted about this user; please review this and retry if appropriate
That's all reasonable. Except.... the system discarded my carefully written moderator message. Pressing the back button didn't restore it, and the system didn't save it anywhere.
In this situation, the system should either save a copy of the message I wrote somewhere so that I can recover it; or else not let me press 'Notify User' to leave that page if it's going to discard my message and throw it in the trash.
(Separately: if one moderator is already on the page and composing a private message to some user, the page should probably warn any other moderator that begins the process of sending a private message to that user.)
(This might be a side effect of Check to see if a new moderator message has been sent within the last few minutes.)