One time I fixed a computer for a lady. Our conversation about it went something like this:
Me: your computer seems to be swarming with viruses and maleware, OS files have been modified and compromised, I believe it should be reformatted.
Her: yes, me and my husband were talking about it, we think it was from the kids going on facebook, we think we should ban them from using facebook anymore.
Me: Just one second
*opens up IE and looks at history*
I am pretty sure it was all the porn sites they were visiting actually.
Her: What? I have two teenaged daughters, they are only 14 and 11! they don't watch porn sites!
Me: well, its either them or your husband since you didn't do it. I would ask your hubby if he did before talking to the kids though, also decide together how to approach it.
Her: are you absolutely SURE it was porn sites?
Me: Well, let me check... ok I just clicked a few and they are definitely porn. Also with names like <insert obvious porn name> dot com...
About a week later I got a followup call, she wanted to clarify to me that they have talked about it and concluded the cause. A few weeks prior one of the girls had a classmate who was a boy over for a couple of hours, and he was left unsupervised in her room for a short while. They concluded he must have gotten on the computer in that interval to surf for porn. She wanted me (the random tech who fixed her computer) to know that neither her husband nor her teenaged daughters would ever visit a porn site...