Previously implemented goes missing after few builds ..Testing

Say there are 1-10 user stories. All tested okay. -> to Production. Then comes the CR with 5 more user stories. All then tested okay. -> to production. Then comes 5 more user stories. Tested play. -> To production.. now here a user story or two from first 1- 10 breaks. Obviously testers will have to carry the blame for the same. Developers have direct access to the QA environments' build path. any developer can go put the code file there. just a simple folder structure. How do we fix this and keep 'our' hands clean? Also Please note that we do ad-hoc testing due to the stringent timelines.