One day my car wouldn't start. I was checking out the starter, and decided to jumper the starter solenoid. Unexpectedy I heard a big boom, like a canon shot. I looked around to see where it came from. Neighbor's yard? Across town? And why is the side of my face sore? Turned out I had created a spark and the top of the battery blew off. I found it on the other side of the house. The top grazed the side of my face on its way up. The battery was just sitting there, pretty as can be, water level normal, but there was no top on it, you could see all the guts of it.

That kind of experience you never forget.