i was a college kid at my first "computer job" doing desktop support for a client in downtown Fort Worth in a fairly nice office suite. Remember those old AT power switches? I wired one backwards, turned it on, and it blew all the insulation off the wires in the case and caught on fire. All in a very nice office, fortunately no sprinkles or fire alarm but I was very embarrassed and nearly cost my boss the client.

EDIT: I once got a call from junior developer who had issued an sql statement on an auto-commit database like "update <table> set x=y; where <some condition>; she fat fingered the semicolon and blew up the whole table. She was crying, I felt pretty bad we were able to get the table restored and back online in a few hours. She still tells me how sorry she was to this day ( about 15 years later ).