Usually, the "We fix out own problems" crowd actually makes things worse when they finally give up and call us, but pretend like they did nothing.
I've had the conversation more than once when the user denies doing anything, "Ok. If you say you didn't change this config, that means I am going to have to quarantine your machine to investigate why it did change. And I *am* going to find out exactly why it changed and who changed it, but when I do it will be four hours from now and I will be a lot less personable, and it will involve a note to your manager about wasting IT resources. Or, if you made the change, you can come clean now. I will shrug, grumble a little, and fix it in 20 minutes, including fixing the original problem you tried to fix yourself, and we can both get on with our day with no repercussions to anyone."
