Yeah I suppose you're right about refund, I think I mean you should try and keep the peace and be diplomatic, because to the householder it will seem like you are trying to make excuses, and they will probably get defensive about their windows!
By the way, what do you do about old traditional leaded windows? I've got one job where you can see where the water has trickled down on thew inside in between the lead and panes, but I thought that they must leak during heavy rain, and just didn't over do the rinsing. I didn't get complaint, but have only done it once wfp, and it rained shortly afterwards!
Any suggestions?