That happens to me all the time, I know i did something, but I must have just thought I did it.
I was describing one of the more obnoxious (and often hilarious) side effects of my medications to Shrek, he almost fell out of the car, he did spray the windshield with Perrier out his nose.
One of the worst "I thought I did it moments" for me was years ago (when I still had a mind!) I had went to the gas station near my boyfriends house and got gas.. $5.00 to be exact. I know this because I only had $5.00 at the time. The lady at the store stopped my Bf's brother and told him I had ran off without paying. I called and swore up and down that I had paid. Sure enough I dug in my pocket and there my 5 bucks was. I was so embarrassed! (I did go back and pay!)