If i would have faced such an issue as a tester first i would try to reproduce the bug with same & different configurations.
In case bug is reproducible for any environment that has been tested defenitely we owe an apology.
In case its not reproducible an explanation about the same will go along..

Initially we check the perticuler secnario is coverd in the SRS. if it is covered we try to reproduce the bug. If it is a valid one. the tester should accepet that. If the secnario is not coverd in the spec we ask the customer to update the SRS.

Hi

I feel every tester had faced this problem at least once in a life un fortunately i too faced the problem in my life but fortunately we convienced the client.

Issue Desc: Report date creteria is not working properly...

Reason: Issue occured if and only if we ran the report in sundays not even saturday.

Mostly no tester tested application on sundays we can deliver all the CRS at end of the week then client will start their verification on sat or sund so found the issue by them this is a very funny issue which i never seen in my life so finally we have escalated the problem to client and he is happy with our explanation.

Srinivas