Be darn careful if you stack round bales.
Six years ago I stopped in at a friend's place to give her a hand with a calf (big dumb calf syndrome).
Anyway, she had her round bales stacked three high under an open sided hay shed, and as I walked by the shed I heard a soft "whoosh" kind of sound.
I looked up in the direction of the sound, and that saved my life, as I was able to jump out of the way of the 1500lb round bale that bounced right where I was about to step.
A bale had come off the top row. It sure wreaked all havoc on her son's tricycle, which was next in line with it's path (thank God the bale didn't let go when he had been riding it earlier in the day).
Anyway, just sharing this in the hopes that others will be aware that situations like this can happen.
We store our bales in single rows, butt to butt in an open hay yard. We also leave a fair bit of room between every four or so rows in case of fire due to a hot bale, etc.
Take care.