Customer decided to 'improve' my sensor placement and now the system is useless

I just set up a full alarm system in a new house last week. The owner called me today complaining that the motion detectors go off all the time for no reason. When I checked, he had shifted every sensor to a different spot because he liked how it looked. He had no idea that moving them messes with how well they see the room. Now I have to fix the whole thing and tell him why his changes caused the problem. It really gets to me when folks hire you but then think they know better. Does this happen to other installers out there?