Kerbal Space Program

This happens every once in a while - I switch to a ship and I am unsure what will happen after I press Space because it is never clear whether what is in bottom stage is ready to be staged or if it only contains already staged parts.

One might think that it's clear - if engines in it have fuel gauges on them, it's spent stage and if they don't it's the next stage, right? Wrong. The game allows activating and deactivating engines regardless whether they were or weren't staged using right-click menus and action groups. And it's good practice to deactivate engines e.g. when docking to an orbital station. Also, xenon engines don't show fuel gauge.

Return to last quicksave is not always a good option either, there might be a lot of time since one was made, or the player may be playing on hard difficulty. The game shouldn't require me to make a quicksave because I'm not sure what will happen after I stage.

If you don't believe, I encourage you to go and play with the two attached quicksaves and e.g. try to deorbit the ship in them, pretending you're an unsuspecting player.

All it really needs is a clear visual cue - for instance a different color used for the "spent" stage, such as in the second attached screenshot.