Dear <client name>
We are able to reproduce the issue reported by you. As this is an exceptional case, unfortunately it was skipped from our testing exercise. We deeply regret for this.
We are going to fix it immediately and will update you once we completely ensure that, the reported case is fixed and no other functionality is disturbed due to this fixing.
To avoid such cases in future, we are going to add this in our test case list and will make sure to check such/similar cases before we upload new version of product on production server.
Going forward, we are also planning to send test summary list to you before we start actual test execution so that, we can make it rich with your suggestions, if any.
In internal team meeting, we have decided to differentiate test cases in positive, negative and exceptional/corner cases to have complete test coverage and to be double sure that, everything has covered in testing.
Thanks for drawing our attention toward this case. We assure you that, you will not find this again in system.
We look forward to strengthen our Quality Checks and cater you best software development service.
Today, before 8 PM PST, we will upload new version of product with this bug fixing and other quality checks relatively.