Yep, I'm with everyone else on this one. You ain't said they have been problematic customers, but good customers for a long time. It can be a genuine mistake or something and so I would go back.
I had one kinda similar. Been cleaning for a good customer for a while and she asked me to clean her daughters as a one off. Her daughter can't afford the price of a regular clean and the mum can't keep paying but the daughter had some work done and it needed tidying. The mum was paying.
I did a cracking job on the house, frames n stone (but painted sills) were mingling. I texted the mum to say all done etc but later in the day I had a call from mum asking if been done as daughter couldn't tell as some bird muck was on the window. I was fuming and went round, to see the daughter on way back from work. I still couldn't see anything up and the daughter came out, apologised as she could tell windows n frames are clean etc, ut she noticed the bird poop!
When she pointed it out I could see it. But, if you stood facing the window as if cleaning it, you couldn't see it. She even acknowledged this, we had a laugh, cleaned it again and all was good. But a genuine mistake from me