Things don't always go to plan.
A few years back, when opening the pool, we found that mice had been nesting in the electrical control box. They'd chewed all the plastic off the wires, creating a massive short circuit.
This year (last Friday) I decided to change the sand in the filter; this involves removing the round black thing on top of the filter (big grey thing), removing all the dirty sand inside, and replacing it with new. All went well, I replaced the top, and turned on.
Water immediately started forcing its way out from the sides of the top, making a terrible hissing noise, so I stopped everything and tightened the two bolts. The same thing happened again, so I repeated my tightening and tried again.
This time there was a huge EXPLOSION and the top flew off with terrific force and water shot out like 'Old Faithful'. The top missed my face by a gnat's whisker.
When we eventually found all the bits, I reassembled them and tried again (you can imagine the state of my nerves by this time). I made sure that everything was really seated correctly, and tightened the bolts. I turned on again, crossed my fingers, and luckily everything was OK. No more explosions.
And here it is; up and running.
Me? I have a few more grey hairs, and my nerves are shot. Otherwise all is fine.