By skyforbes An example of the most unforgiving is in the original EGA Space Quest. There is a piece of glass you need to pick up that the game doesn't really draw attention to. If you don't have it when you reach the point you need it, you can't go back to get it so you had better hope you have a save file you can use to get it. When it was later remastered to VGA that piece of glass was given a glint animation, so hopefully you notice it and pick it up but it's still easily missable and just as bad if you don't pick it up.
An example that's a bit more forgiving comes from the same game. Right at the beginning, a scientist dies and, as his last words, tells you the name of a file. The file is in the very room he dies in, and you practically can't miss how important this file is. HOWEVER, you can still not get the file and thus be unable to beat the game properly. With the scientist telling you about the file so dramatically, it's really on the player if they don't bother getting the highly important information.
Then there is the most forgiving, seen in games like Monkey Island, where if you reach a point where you need an item, you are sure to have that item, or you'll still be able to go back and get the item. If you get a game over in these sorts of games it's not because you missed something you don't have access to anymore.
Personally, I like the second option, but I can understand why more games shifted to the third option as time went on. But what if someone made a game in the style of those old games. Would you want them to do the Monkey Island thing of make sure you can't save the game in an unwinnable state? Or would you be okay with a more unforgiving game? How unforgiving?