Heck, everyone who is sensible expects the unexpected from their animals......and even an electric fence is never 100% safe unless the electric never goes out at your place, or the battery never runs down or your solar unit finally gives up the ghost!
No one can trust any method to "protect" your animals, as many posts on here verify.
You also never know if your next door neighbor, usually a calm, unassuming man, will snap and murder you all in your sleep!
You just monitor behavior patterns, try to subvert any potential deviations from the normal pattern and then you hope that nothing happens to make your normally well-behaved dog snap into a blood crazed wild animal.
Just like taking said dog out in public. You may feel like you are safe with your dog on a leash and "supervising" any encounters with other creatures such as children, other dogs, cats, etc. But it only takes a millisecond for that dog to snap at someone and rip open a vessel. That's all you have......the hope, from past experiences and patterns, that your animal will NOT rip a child's face or throat before you can pull him back....especially a very large dog.
If one were to let fears of what could "potentially" happen at all times, one would never set foot out the door....nor let the dog do so either. Life is about living....with reasonable expectations that your judgement is pretty sound, but realizing that nothing is 100% in this life, except that we are all going to die. Chickens die. Dogs die. We all die. Get over it and enjoy living with the dog you trust, the neighbor you trust, the chickens pecking through the open spaces....