What happens when a site goes in maintenance mode while a user is editing an node content? Let's imagine a user is editing a node content. In the mean time, a Drupal administrator puts the site in maintenance mode.


Does the user have a chance to save his work (assuming it does
not have the "Use the site in maintenance mode" priviledge)?
Is there a way to send them a warning, like 'maintenance in 5 mins'?


Thanks.
Unless you notify the editors about that, they will lose their work on that node. You can do that either by using some contrib modules or by creating your own custom module. Either way, I think you definitely need some kind of ajax solution so they can be notified even when they are on the node edit page.

You can check Privatemsg and Private Message Growl modules. I'm not quite sure how they work but they look promising.