Finished up something on the computer this morning, went off to shower.
My towel was wet. "That's odd," I thought; "I wonder what I used it for this morning?"
Got into the shower, stood in the water, wet my hair. Ran my hands through my hair.
"Weird," I thought. "My hair doesn't feel like it normally does when I'm about to shower. It's kind of smooth."
I put some shampoo in my hand, and suddenly got déjà vu. As if I'd--
"Wait," I thought. "Did I shower already this morning?"
I thought about it and realized that in fact I had. I had finished showering, then I'd gone into my bedroom to get dressed, and gotten sidetracked into something on the computer, and I guess I neglected to set my mental "showered_already" flag.
I'd forget my head if it wasn't screwed on. As my parents used to say. At least, I think it was my parents; I don't remember where I first heard that.