Yeh, thi does occasionaly happen. I just point out that it's on the inside, politely! and usually they retreat with "tail between legs"!
Had one last week. New customer, old, well to do lady, known me from years back. Apaint mark smack in the middle of a pain! I saw it and realised it was inside. As I moved on to the next window, up she gets, walks over the the window peering at the mark, opens the door (next to the window).
"David? I think you've missed a bit here?" She says.
"No, it's a paint mark on the inside" I reply.
She feels it and realises. "Oh yes, I see"
"I'll scrape it off, iff you like" I said.
Popped in ,scraped it off, everyone is happy!!!
David