Year ago in university.
We opened our university's website and select inspect element in browser then edit the header tag to "hacked by..."

My friends and i : hey look, we just hacked university website.

Our friend : oh let me see, damn you, how did u do that?

Our : it was easy, just don't tell anybody. He answered ok.

After couple days our proffesor asked me : do you know who hacked university website? I want to know if anyone could hack it.

I answered: no sir. I don't know.

I think our friend still thinks we hacked the website xD