I had a very strange experience today. As you know, I'm a couple days in cold turkey with Colemak, which is actually going pretty well.
Anyway, I've experienced the "unlearning" of Dvorak. I don't know why, but I have found that going back to Dvorak immediately is completely impossible. I'll either type nonsense, which in my mind is correct, or I'll shift into Colemak.
Anyway, maybe an hour's worth of "relearning" Dvorak has solved the problem. It wasn't so much remembering where the letters are, but rather how to move your fingers around from memory to get the desired text.
This also happened when I went from QWERTY to Dvorak -- and at the same stage of learning the layout (between knowing the letter positions and speeding up). It was less severe with QWERTY -> Dvorak, and switching to Colemak left my QWERTY skills unchanged.
I'm thinking, somehow in my mind, when I'm learning a new layout, it blends with layouts I already know. It's as if I have to sort the layouts out in my head to use them all.