I have tested a web application. I have tested it by considering all positive and negative cases. Even my project manager has tested the application. When we decided that everything was going fine, then application has been moved to live. But now customer has found a defect which was not found during testing process at our location. At test environment the results were correct. There was no problem at test environment. When the application has moved to live, the results were shown wrong.Don't know where was the problem.
What should a tester do in this situation. what kind of explanation should be given by the testers. and who will be responsible for this problem, the tester or the developer?
Thanks & regards