it was not intentionally, turned up today 4 weeks to the day to do a clean, I am looking at the windows and they all look good except one that stuck out like a sore thumb, it was one pain of glass on a 6 paned window, I thought something must have been spilled on it on the inside, and that is unusual as she keeps them spotless, then thinking back she owes the last clean, she normally pays quickly by bank transfer, anyway I clean the windows and get to offending pane and clean it and low and behold it was the outside.
I must have soaped it up and never squeegied it off

I haver never done that before and I can't think for life of me how I got distracted enough to miss it, no wonder she never paid me

don't blame her really, but she could have phoned and told me, I don't blame her for not paying, and indeed I will not ask for payment