Some users might, if a little tired, be tempted to auto-fill the master password. A little easter egg could be fun (could be something as sober as “You can’t auto-fill the master password“ or something more like “You might need some caffeine“)
Some users might do this even when they’re not tired at all (for their own account or for e.g. an account of their relatives that they help to administer). So, what about those users?
Oops, I see the issue… Maybe only for the extension instead then?
… or when logging into the web vault.