From the I can't believe I had to write that file comes this question from a customer:
Our customer is modifying the ABC.DLL file that comes with Windows in order to accomplish XYZ. Is this supported?
No, of course this isn't supported. I can't believe I had to write that. if you modify a system file, then the thing you're running isn't Windows any more but is rather some sort of operating system that resembles Windows in many significant ways.
(Imagine the extreme case of this: The customer modifies NTOSKRNL.EXE, KERNEL32.DLL, USER32.DLL, etc. so that they happen to be byte-for-byte identical to the files that shipped with Windows 2000. Does this mean that Microsoft supports Windows 2000?)
The customer is just looking for an official Microsoft statement that this is not supported. They know that it's a bad idea, but their client wanted to have this feature, so when the customer tells the client "No, we can't do this," they need some sort of justification.
Read more: The old new thing