Try reading what I posted again. I posted "if they tell me", simple enough I would've thought. Does a job occasionally throw something unexpected at me? Yes of course they do. Can I foresee every problem lurking in a carpet? No, can you?
Once I've given a quote thats it, I don't play cowboys and indians with people. If I've not done my job properly and enquired "have you cleaned the carpet before?" (nice simple question which is normally answered honestly) then I've no one to blame but myself.