To set the scene, this event took place in my first year at CH, when I was 11 years old. (Digressing slightly, the nickname for a first year was "squit", which is apparently Latin for "inferior being".) One day there were a few of us standing around in the kitchen: a random cat had wandered in, so we'd put down a saucer of milk for it. I was holding a butter knife that I'd brought from home, i.e. a knife with a metal blade but just rounded edges rather than a cutting surface. This was back in my pre-vegetarian days, so it had a bone handle.
As I stood there watching the cat drink, I was idly swinging my arm backwards and forwards. There was a kettle plugged into the wall near me, but the plug wasn't quite snug: it was in far enough to get power, but there was maybe a 5mm gap between the plastic surfaces. (If that doesn't make sense, let me know, and I'll try to do an artist's impression.) The knife in my hand slid in between the plug and the socket, and knocked gently against the top metal pin. Nothing happened, but I was vaguely aware of the collision, so I shifted my hand slightly so that I was swinging the knife a bit lower. On my next swing, I knocked against one of the bottom pins; again, I felt the minor impact, but nobody else noticed.
Rewinding a bit (yay for non-sequential storytelling!), back when I was a little kid in playschool we had various toys to play with, some of which were intended to be educational. In particular, there was a hollow plastic thing about the size/shape of a football, with various holes cut in its surface. The idea was that you had a bunch of plastic objects, and you had to fit these inside the ball, but each shape would only fit through one of the holes: the round peg through the round hole, the square through the square hole, and so on. I quite liked this game, so I played with it a few times even after I'd mastered it. Maybe this was an early sign that I'd do well with maths/computers, since I liked the elegant way that things had an exact solution.
Anyway, flashing forward to the kitchen again, I wasn't paying a great deal of attention to my arm movements, but one small corner of my mind thought "Hmm, if I go up then I hit something and if I go down then I hit something. I wonder whether I could fit between the two obstacles?" It turned out that my knife was quite a good fit, so it slid neatly into place, thus completing a circuit between the upper and lower pins.
This produced quite an impressive shower of sparks, much like a firework display. The cat leapt off the working surface in terror, and the light went off. A few seconds later, other boys came pouring out from the studies along the corridor, because all of their stereos had suddenly stopped working at once; I'd managed to blow the fuse and knock out power to a large chunk of the boarding house. As for my knife, about half of the blade had simply vanished, and there was a blackened edge left across the stump I had left. It's probably a good thing that the handle was insulated, since if I'd been gripping the metal at the time then I expect I'd have got a bit of a jolt.
I still maintain that this was an easy mistake, which could have happened to anyone, but I haven't had much luck convincing anyone else of this!
A few days afterwards I had to go to The Counting House, to explain what happened. This was my only visit there, so I'm not quite clear on the big plan, but I assume that it was intended as some kind of guilt trip. In other words, rather than just getting fined for kicking a football through a window, you'd have to deal with the consequences of that action by facing the people who would be responsible for repairing your damage, and so hopefully you'd be more careful in the future. In my case, the accountant (?) I spoke to seemed a bit dazed by the time I'd finished my story, so he basically just said "Oh well, try not to do that again, eh?" and I was happy to go along with that.
On a vaguely related note, I see that Sky are recruiting for a new gameshow: "Are You Smarter Than a 10-Year-Old?" (I've heard of the corresponding US show, which presumably inspired this: "Are You Smarter Than a 5th Grader?".) I think I'll give that a miss, because it might be embarrassing to find out how much I've forgotten, but I'll keep an eye out for it when it comes on TV.