The King's Quest games were absolutely brutal. Actions performed early in the game could prevent you from winning at the end. I found this list: http://www.thealmightyguru.com/Wiki/index.php?title=Unwinnab...

The only reason we beat these as kids was having a near infinite amount of time to waste.

ADOM had similar issues. You kill a cat any time, you miss out on a later quest. Instead you just had to fight the cat lord. Missed out on a very powerful ring.

And it was so, so easy to kill a cat. You use the automatic repeat-move key and auto-hit when you collide with a monster, and if you ever find a cat anywhere that way, its dead. People would rage-quit and start all over, it was that frustrating.

King's Quest..... That one was a challenge. As it was easy to miss an item from area and not be able to return to pick it up.

I can attribute my debugging skills to playing these games as a child. As you had to be meticulous.