I've had a few but one of my worst was not long after I had joined a new mobile phone company called Orange in 1994.
I wrote some code for an exchange in London (global title translation for anyone who's interested) but I entered 1 digit incorrectly....
The result was as soon as the code was entered, it caused an infinite loop between several exchanges which took down the network in London.
The outage was that bad (12 hours) it made the front page of The Sunday Times
In my defence, I did tell my boss I thought it was my code but he didn't believe me so was chasing the wrong thing for most of the day.
Still, it wasn't as bad as the guy in Milan, Italy who removed the working CPU instead of the spare whilst I was doing an upgrade of Omnitel's network, and took out northern Italy for over half a day.