Many years ago ...
... when I was the IT manager for my company (not have gratefully passed that poisoned chalice to someone else) I had been configuring a new Linux server which was being used to host a couple of fairly important applications - e-mail and a Document Management System. Before going off on a nice two week holiday to Canada, I had been tweaking a couple of settings relating to the LDAP directory, and had accidentally left it in a state where the server would not boot unless it couple first contact the LDAP system, which of course was also hosted on the server. Was not able to test the settings to find the problem since we only had a production system at that time; my company was too small to afford a development/test server.
Anyway one fine sunny morning I & my family was just visiting the Montreal Olympic Stadium when my mobile went off. It was my Director who explained that they had had a power glitch that had forced the server to shutdown, and it was now not rebooting - could I please help them sort it out. This was definitely my personal "Oh Shit ..." moment! For the next 90 minutes, while wife & children went off to see what they could see, I was stuck outside the stadium diagnosing the problem, realising what I had done, and then having to remotely guide my boss (who I should add is a pretty good guy - have known him for over 30 years) on who to boot the system into run level 1, mount the file systems, edit the faulty configuration file, and then elevate the system to run level 3.
After that I persuaded my boss to purchase a small development/test server; I was also a lot more careful when editing configuration files after that!