Sometimes I daydream and forget which windows I've done. Its scary. Window cleaning is so repetative you sort of hypnotise yourself.
When I'm going for it I have no problems as I'm always thinking of my next move but when I'm chatting to customers whilst working, I sometimes get engrossed in the conversation topic and forget if I cleaned a particular window. Then I have to walk back and inspect the window to double check to put my mind at ease.
If left alone for long periods of time with no distractions on large jobs, I also feel like I'm hypnotising myself because I'm so relaxed, calm and peaceful with my thoughts, daydreaming and singing in my head.
I can't think of many jobs that allow you the opportunity to drift off in your thoughts whilst earning a good wage at your own pace.