We had Windows machines in around 94. They booted to some kind of menu where you could do various things I don't recall...and boot Windows. There was a problem with my friends machine one day and I saw the IT admin type a ludicrous single character username and password to get into this boot menu.

So I used it to have a look around and change the startup message to insult a friend. Then I told him how...and he told everybody. Next day all of the machines had obscene messages, and someone was caught doing it, they said my friend told them, and he told them that I told him. Thanks buddy.

I remember being terrified as I told the deputy head how I 'cracked the code', but I think I just got a detention.