While anything can happen, it is really unusual for free ranging chickens to eat anything that hurts them, creepy crawlies or plants either for that matter. Part of that is that things that hurt them generally do not taste good. Since they free range they have other options. Part of that is dosage, one bite won't kill them. For practically anything harmful they'd have to eat a fair amount to harm themselves. Since it doesn't taste good and they have other options they don't eat that much. Their digestive juices can neutralize some stuff too.
An example. The leaves of a tomato plant contain a substance that is harmful to them.. They taste awful too. It takes a bit to harm them but it is toxic. If your free ranging chicken happens upon a tomato plant they might take one bite, shake their head, and move on. If you keep chickens in a bare run so they never get anything green and toss a tomato plant in there they might eat enough to harm themselves. They don't have other options.
They eat things we consider totally disgusting, spiders, maggots, slugs, whatever they can scratch up. Those are actually full of good nutrition. Don't be surprised to see yours catch and eat mice, frogs, or small snakes. I've seen them cut those into bite-sized pieces by pecking with their beak. They seem to live by the motto "Do lunch or be lunch". If it doesn't eat them they will try to eat it.