Not sure this was really my fault, but I was setting up some file servers and turned on 8 machines at once, and caused a dip AC power line which was bad enough to fry every single hard drive. These were $25,000 machines that each had 5 2-gig SCSI drives which were worth $2,000 each at the time. Total damage was about $80,000. The worst (best) part was when the repair tech replaced all the drives he did the same thing I'd done. $160,000 in total damage isn't quite your $300,000 threshold, but it was real damage to production environment equipment and actual repair dollars that had to be paid.
They upgraded the power lines the next week.