This just happened to me yesterday! I use a three corner cycle in PLL that I made up one day, and I use it every day with out thinking about it. Suddenly yesterday the muscle memory was gone and I had no clue what the alg actually was. Pretty sad that I actually had trouble sleeping last night because I was trying to find it lost wherever it was in my mind. I finally figured it out this afternoon after realizing that I knew it by watching how certain pieces move around the puzzle and not in a muscle memory movement if that makes sense. Some are muscle memory moves, and others are very visual.

Anybody else have this happen? .....and drive you mad when it isn't a normal alg you can just go look up?

That happened to me a couple days ago, I was solving my Volcano by first starting with the tips, when I got to the last two, I lost what I was trying to do, it took me an extra 20 minutes solving that part.

Fortunatly it has never happened to me
Or maybe it happened me with the corner-permutation algorythms of the Skewb, but I didn't solve it for a lot of time, so I guess it doesn't count...

I have 3 algs that I forget about once a month. Sometimes they come back on their own, occasionally I figure them out again.

OMG The exact same thing happened to me last week. I forgot the alg that I use every solve. I was so mad that I whipped at the wall as hard as I could. The cube blew up and I was satisfied. Two weeks later I reassembled it and solved it. thank God. I'm glad to know that I'm not the only one.