It cost me a pool liner. When we bought the house in 07 it had a 24ft above the ground pool. They had dug in the side of the ridge and shored it up, about 4ft, with rr ties. After 2 liners, couple pumps and all the kids leaving we got rid of it and turned the area into a garden spot.
On occasion if I want to shoot off a few rounds, new gun, new mag, new ammo, new itch or if one of the kids want to shoot. 
We have our 5 yr old g-grand-daughter by adoption. We got her  little 10ftx30in frame pool last and put it in the old area. I still had a one lane firing line and hadn't had any issue until today.
My oldest granddaughter, the little ones mother, also lives with us.(It's a whole different story, our daughter, the grandmother also lives here.)
Any way, I told Abigail if she'd load up some mags with my new speed(not) loader I'd let her run one thru the new gun. We go thru a function and safety brief on the gun. She had never fired a AK style weapon otherwise she's square. I had a gal jug on the ground in front of the embankment and away from the pool. She was getting a feel for the gun, she's ambidextrous with shooting. I told her the firing line was ready  and don't shoot the pool. BANG! The side of the pool slit open about half way down. I laughed so hard I nearly pissed myself. The other 14 went in and around the jug.
The timing was perfect for my last word and her shot. The frogs were not happy with losing half the water.