Backstory: At my former residence, I had a stray that would come around and pester my kitties that liked to hang out in the window perches. Occasionally he would jump up and grab the screen with his claws, which would scare the crap out of Prissi. She went nuts when he would come around. She'd attack anyone in shouting distance, running them into corners, biting me if I tried to calm her down, etc. She was a terror.
One night Blackie came around and got hold of the screen. Prissi was her usual crazy self, and she pushed from the other side and the screen gave way. She was halfway out the window, and I literally had to close the window down on her, to keep her from getting out. She has no claws (I got her that way!), and it would have been an ugly duel, I fear. I still thank God that by chance, I happened to be standing right there.
As an aside, I had to put Blackie down months later, as he was injured and FIV-positive.
I now live in a 3rd floor apartment, and have two big bedroom windows, and a picture window with a balcony, where birds and squirrels reside and make excellent eye candy for the babes.
The kitties love their fresh air, but there's no way in hell I'd ever think of leaving them alone all day without having baby gates in all of the windows. One each of the smaller wood-framed ones in the bedrooms, and two stacked upright in the picture window. The wood-framed versions have thin wire screens, and offer excellent visibility.