Guilty Conscience?

One of my colleagues send a stern email out to students on his course, reminding them about plagiarism and how they should always do their own work and not use work by other people, and any sources they do use should always be acknowledged in the documentation.

He got an emailed reply back:

Is this because of me?

He sent this reply back to the student:

Why? What have you done?

He hasn't heard from the student since...