I'm familiar with this message when a command's output is more than one line.
However, on one of the systems I use, I see it every time I write with
:w, despite having plenty of horizontal space for the message. For example:
This only seems to happen when I use vim in a tmux session, but I can't work out why this should happen. Every other aspect of vim and tmux seems to be working fine. Turns out it has nothing to do with tmux.
Update: I tracked it down to the following line in my vimrc:
Using a vimrc that contains only this line is enough to trigger the message when writing to a file. Interestingly, it only happens when I write to an existing file.
Why would this happen, and on this machine only?