Problem always occurs at customer side because customer uses it maximum.
We as a developer building things regularly and test it fast and in hurry. Also we can’t test it for all use cases that customer is going to execute in practical.
From last couple of days i am driving a car. There is a problem in breaks. It makes sound when i try to control it after running at 80Km/h. But when i consult technician, technician took a test drive and problem reproduce after several attempt and the behaviour is not exactly same as i was facing. So my point is this type of situation occurs in each discipline. A patient knows what a pain is, doctor doesn’t know. Doctor simply tries to understand the behaviour and diagnose it.
In software development also while developing software it works perfectly at developer front. But when we ship and after some times customer will asks for specific cases or complaints that are only arise at customer side only. When we as a developer tries to reproduce same problem it may or may not be reproduce exactly in same fashion.
In conclusion we must have to listen our customer carefully and tries to provide solutions for their complaints.And our solutions should makes them feel happy in use of our services or products.