I think my best screw-up as a kid was an attempt at a "bomb". At a local festival there were these confetti things. Carefully take off the top part of an egshell, dump the egg, fill with confetti, then glue tissue paper over the opening. You would then crack this "egg" over your friends heads and cover them with confetti.
Little pyromaniac me thought it would be fun to make one explode by filling it with gasoline and lighting it. Being careful (as always!), we set the thing out in the middle of the road so it wouldn't catch things on fire. Lit it, and it just sat there burning like a candle. How boring! Then a car with two elderly people drove up and stopped, looking at what was in their path, with us kids watching it intently (I wouldn't have driven into it either!) I decided we should put it out so the people could drive on their way. So I went out and stomped on the thing. Bad idea. The gas went everywhere and the road was afire. With me in the middle of it. Somehow I managed to escape without any burns what-so-ever. A miracle. Eventually the road fire died out, and the old people just slowly drove on their way, shaking their heads at us.