Impeccible
I used to be extremely good at spelling. But sometime around ten years ago, shortly after college, I started noticing that I no longer always knew how to spell any given word. (I was particularly starting to slip on -able vs -ible, though yes, I do know how to spell impeccable. Title is joke. Funny! Ha-ha, not strange. Though maybe strange too.)
I also used to be an extremely accurate typist; I made mistakes, but I almost always knew when I'd made a mistake, and would correct it immediately. In the past few years, I've started slipping about that as well; more and more often, I'll type something that I don't think contains any mistakes, but on later re-reading I'll find half a dozen glaring typos.
The weirdest part, to me, is that my worst slips recently have been homonyms. Tonight I was retyping something from one computer to another; it included the phrase "choose or create." I typed it, went on to something else, came back, and found that I'd typed "choose are create." And that's not even a homonym, just a vaguely similar-sounding word.
My fingers have usually been pretty good to me, but they're fallin' down on the job. Have to send them to a remedial typing class or something. Unless it's the brain that's the problem. Which is more than likely. You do your best to keep all those cells alive, and they go and die on you anyway. Hmph.