I was called over by a colleague. She needed help because her computer kept telling her that she did not have permission to run certain programs or access certain files.

She logged in to Windows in front of me. The first thing that I noticed that the username was her office email address. I asked her about it.

Me: Why is your username your email address?
Her: It was this way when I got it.
Me: That is impossible. I made every Windows installation here and I always use the same username which is [companyname] as it is our policy.
Her: I'm telling you, this is the way it was when I got it.
Me: Are completely sure?
Her: Well.... someone else must have renamed it.
Me: So someone fired up your laptop, used your password to log in and changed the username to your email?
Her: I don't understand it either. Is it possible that it happened accidentally, on its own?
Me: ...

Then I explained to her that changing the username on Windows 10 may result in problems with file permissions.

I am not mad because she didn't know about this. I am mad because of her idiotic lies.