Chirpchickens, some rabbits LOVE chicken feed and will leave their own feed alone to eat the chicken feed. That can result in a done, dead bunny. The most common problems would be kidney failure from too much calcium and protein, or GI stasis from nowhere near enough fiber. Even in small quantities it can be damaging over time. So how much rabbit feed does she eat in a day? She could eat that much chicken feed in a day and if she kept that up for a couple weeks she could keel over, because it's not about "how much" but really "what percentage" of her diet is chicken feed. A few pellets because they're there and she's bored won't cause any real issues... If it becomes a significant part of her diet, it can be serious.
A good brand of pet rabbit food that I feed to my pet buns has 0.80% calcium and 12% protein. Given a good %14 hay that's equally low in calcium that's more than enough for a non-breeding rabbit (breeders need more, tends to be up to 2% but not over, just for milk production and growing kit bones, nothing else). My chicken feed has 4% calcium and one of the feeds I have has 20% protein. Imagine if you ate five times the amount of calcium in a day than your body could absorb and nearly two times the protein you needed? And then it all came out crystalized in your urine. And then those crystals get stuck in your kidneys and the kidneys shut down from it and you need a kidney transplant. That's what happens in rabbits, but there's no rabbit organ donors out there... So the rabbit just dies.
Alternately, rabbits need LOTS of fiber (20%-30%) to have good digestive function. If they don't have it, their digestive tract just stops moving completely and it's pretty much impossible to start back up. Chicken feed has VERY little fiber in it. Less that half what they need to survive.
That's just one of a dozen reasons why chickens and rabbits don't mix. It's by far the biggest one though.
Amen!!! Perfectly said. If you want to have your bunny for a long time, don't ignore what Chocolate is saying. It is very good advice.