Have you ever forgot the solutions to your own levels? I'm sure if you had made a lot of levels, you're bound to forget how to beat one of them. I forgot the intended solutions to some of my levels, that I ended up beating them by finding backroutes, that I didn't know was there.
Oh yes.

I made a Lemmings 3 level where you have to build a staircase up the side of a giant snake, as well as rescue the "swallowed" bonus lemming inside. All the way you have to put all your bricks and shovel strokes in exactly the right places. I don't know any more what those places are

. I still hold out hope of finding my sketch of that level.
There's also "The Floodgates Open" where the time limit completely threw me when I went to solve it later.
I have no idea what the original solution to Lemmerixo was. (The original Cheapo version, from Lemmings Plus 2). The solution you see to the LP DOS Project version existed in LP2 only as a backroute; I do know the original solution was *very* complicated and definitely up there with To The End and other notable levels of mine.
I have made only 3 levels where I forgot the solution.