What's the most expensive thing you've ever broken?

I was working on the roof of my garage when my oldest son (10) asked me if he could mow the lawn. "Sure" I tell him, "the gas is in the can on the shelf there" and off he goes to the front yard to gas and start the lawn mower. I had just bought this mower and it's been used once, I figured he couldn't do much to hurt it as I hear him plowing through the light grass in the front yard.

I finish on the roof and hear him pulling at the new mower in the front yard, it won't start. "You put gas in it right?" I ask, pointing at the gas cap, "Yes I filled it up" he says, pointing at the oil cap.

I flushed out the oil/gas mix in the crankcase and think I saved the engine, my son spent the rest of the day embarrassed every time I told the story.

I would have lost about $200 on the mower had it died, I think we might have taken a year or so out of it's overall life, but it worked fine for the rest of the day, and is now stored for the winter...fingers crossed.

Does anyone have tips on what else to do if that happens in the future? I have one more son and a much younger daughter.

EDIT: I should add that I only told a few people, I think he left learning where gas does and doesn't go in a lawnmower. I told him the story about how I broke the points on a brand-new Honda XR80 when I was a kid....I traded an Atari for that bike and I never got to ride it because I couldn't afford the replacement.

I don't think I emotionally crippled him or anything, sheesh.