[G Griffin]

GB,
Those "hard water marks" are very unlikely to be your fault. They are stains caused by mineral deposits. There`s usually something near the glass- eg.stone, mortar- that will be washed onto the glass by rain more than your pole.
Glass, like anything, can get stained over the years. I`ve heard it called concrete cancer.
Just explain to the customer and you could charge to try and remove them but I wouldn`t worry, it`s probably not your fault. 

[Dave Anderson]

I have caught myself out a few times and it's normally during the transition from Winter/Spring into Summer.

During winter/spring I tend to be a little less attentive and quicker and during the warmer/hotter summer I spend more time on the glas and frames with more flow...I have been caught out before...