I thought I was imagining things, but I have had this happen to me before too. I wanted to build a campfire and I KNOW I learned the DIY but I searched my list of DIYs (I even ordered them alphabetically) and I just could not find it in my list. I tried buying them off someone and when I had the recipe it said I already knew it. When I looked at my recipe list there it was again like a phantom.

I thought I was just blind or really dumb, but it kept happening. Thankfully they came back and I didn't have to relearn any of them, though (yet).