I was working in an office/ reception job. The manager intended to let me know he planned a fire drill.
Cue the alarm going off, my voice over the intercom with the safety instructions so everyone was outside the perimeter of the premises all shivering our asses off when fire brigade came down the industrial estate heading for our unit.
The manager came over put his head on my shoulder and whispered - you called the emergency services, didn't you? Yes, he had forgotten to tell me it was a drill.
The emergency services asked where the fire was, glad to be told no damage or injuries.
Yes, I was only doing my job but felt embarrassed at wasting emergency services time. 
The next drill I got a 10-minute warning so I didn't call them.