Oh Lord, so much, usually early doors. I have more than once sprayed hairspray under my arms, deodorant on my hair. I have used peroxide based contact lens cleaning solution instead of saline when putting them in - I had to have a day off work for that one. I have showered, put on a dressing gown, gone down for breakfast, wandered back up and got back in the shower. And I regularly forget whether I've cleaned my face or not so have to do it again.
But my friend had the best tale (not strictly about forgetting, but it's related to medical matters). She was dressed in a fetching tie-up-the-back hospital gown when in labour and suddenly realised she'd lost her lovely diamond engagement ring. Panicking, she hauled herself off the bed and bent over to look on the floor. At which a doctor reached down and removed the ring from between her buttocks.