been there done that:
Leaking tank 120 gallon! 50% of it on the floor, good thing it was cement floor.
Heater crack, killed all fish in the tank! just last week I loss 8 Ppetricola due to bad heater!
Power Outage: luckily it was only 1 day, all my fish survived because they were grown.
Bad acclimation killed my priced Male Yellow Blaze.
A 300 gallon show tank exploding, would be my biggest fear.