Possibly Jason is doing/has done this as a part of "run button improvements". Or maybe not.

steps to repro:
1) create a plan that contain two stages. Second stage let be manual stage.
2) run that plan, don't run the manual stage.
3) modify first stage's job to run script task "sleep 1d".
4) run that plan (#2 time)
5) while it is running go to build no #1 and click on 'continue stage' button (on the left navigator).
6) you will be shown the error as in attachment (assuming you have concurrent builds set to 1, or none at all)
6a) where I had expected there will be some links to either CAC documentation ("what is the concurrent builds? how can I change it?"), or either to concurrent build configurations (see BDEV-626 for the urls).

What's the problem with the above message (real story from Gdansk office jira devs):
If the user is a developer that doesn't have wide knowledge of Bamboo he might encounter this message when trying to 'deploy' one of his manual stages that had finished in the past. And if the plan is running that developer has no idea what we're trying to tell him ("what concurrent builds, I see some red error, get me out!").