After Completion of final testing build will move in to User Acceptence Tesing.in User Acceptence tesing if client found any defect based on that defect priority and severity client will decide wether to fix the defect and then move to production with in the specified time lines or else they will postpone the production roll out of the build
It all depends on the severity of the bug found. There might be instances, where the bug might not be severe i.e. it might not hamper your functionality but still has to be fixed at the earliest i.e. the bug is a high priority bug.
The defect found at the customers end is analyzed
1. If the issue is being reproduced in the Development/QA environments.
2. If the issue was not in the regular work flow(as per initial requirements)this would go in as a change request.
3. If it is a severe/priority defect, then the possibility of a patch might be considered.
All these changes/fixes have to be tested prior to sending them for acceptance testing.