Zeraan: In Link (Zelda II), I couldn't find an item in one palace, so I thought there's nothing there, and proceeded to kill the boss there. As you know, defeating the boss then exiting stage right results in the palace being turned into rubble.

This made the game unwinnable because now I don't have that vital item to get past an obstacle. I had to start over, and eventually I found the secret wall that you can walk through...

Protip, don't destroy a palace until you got its item...