Oh, I just remembered another saving instance, but it kinda saved me. I was starting the Water Temple in Ocarina of Time and I saved right when I started, just in case I raised/lowered the water level at the wrong time. But by the time I actually did mess up, I felt like I had accidentally saved at some point in the dungeon and I couldn't remember if I had saved at the beginning of the dungeon because I had left to go eat lunch. So when I came back, I had no clue. I turned off the game, knowing that if I really didn't save, I'd have to get a bunch of heart pieces again.
Lucky for me, I did save at the beginning of the temple. So I guess this isn't forgetting to save, per se...just forgetting if you remembered to save. XD