I use a 48” high electric netting for my chickens. A couple of times I’ve had mature dual-purpose hens fly over it. But I practically never have any get out, these got out by going out of a gate when I got careless. They easily cleared that 4’ high fence with a lot of room to spare when I tried to herd them back to the gate. Chickens don’t herd easily.
When I raise adolescent birds with the flock I do occasionally have birds get out, usually young cockerels. They get in their fights and the loser goes vertical to get away if he is trapped against the fence. Sometimes he comes down on the wrong side. This used to happen pretty regularly but I’ve learned a few tricks to help.
If I build sharp corners into my netting they get trapped more often. Now I use flattened corners. In your backyard fencing you probably can’t do that, property lines and all that. I also discovered that if I build it fairly wide as opposed to long and narrow fewer get trapped. Your back yard should be pretty much like that so this should not be all that big a problem, even with square corners. This kind of stuff can happen, but if you are not raising adolescents, especially cockerels, it really doesn’t happen often.
A very common way for chickens to get out is by perching. Chickens love to perch, just for fun. They can fly up to some pretty high places. If the top of your fence looks like a good place to perch, say it is solid, it’s not all that unusual for a chicken to fly up there for fun. Who knows which side it might fly down on? I don’t know what your fence looks like, but the fix for this is to have wire fencing stick up several inches above the top of your solid fence so they don’t see a good place to land. I’ve done this and it can be done in a way that it doesn’t look as ugly as it sounds.
All this does not mean you will never have a chicken get out. I don’t give guarantees for chicken behaviors. You might have a building or tree limb near the fence they fly up to perch on. I’ve had a couple that once they learned they could get out of a run with 5’ high wire fencing, they flew out daily. But people keep chickens contained in fences only 3’ high. The chickens can get out but do they really want to?