I think we've all done that! Only I reckon I've done it more, (and I don't want to sound like 'Billy Two Sheds' here).
Once, while up a ladder, wind blowing - a woman shouted to me and asked me to clean her windows. We negotiated a price and she left. I knew the area very well.
I turned up to clean, a little suprised it was a bungalow, but cracked on anyway.
They weren't in. I came to collect. They weren't in. Every time I call for my money and receive no answer, I leave a 'Your window cleaner came to collect' certificate. I call twice and on the second time leave a 'It will be added to next month' chit.
Well, I cleaned the place three times, over three months and one day received a phone call. (I put my telephone number on all my stationery.)
The call basically said, we didn't ask you to clean our windows, and we're hardly home anyway. Well, I was polite, but did state that they could've phoned me sooner than let me clean their windows for three months.
Wor lass went mad! She would've bricked their windows if I let her!
So, I doubt there's many here that's cleaned the wrong house three months in a row - apart from me. I am a Geordie mind!