Why I am not able Turn Off CustomErrors?

This week I got an interesting issue altogether. One of my customers was unable to turn off the CustomErrors at all. We checked all the web.config files in hierarchy including the web.config and machine.config in C:\WINNT\Microsoft.NET\Framework\v2.0.50727\CONFIG. All files had an entry <customErrors mode=”Off”/>, purposefully added to avoid this behavior. But we were still getting below message…

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