Rabbits are grazers, and a rabbit's gut is delicate. A food that is fine for you, or fine for a dog, can give a rabbit a sore tummy or worse. Welfare groups agree on the big picture: a rabbit's day is mostly hay, with a small handful of leafy greens and a little pellet food. Everything else is a treat or a no.
Tap a food:
A rabbit's gut depends on a steady stream of fibre and the helpful bacteria that live in it. Sugar and starch (bread, cereal, biscuits, yoghurt drops, even too much fruit) can upset that balance. The House Rabbit Society chapter in San Diego links these foods to a serious gut illness called enterotoxaemia, which can be fatal. That is also why muesli-style mixes are on the "better not" list: rabbits pick out the tasty bits and leave the healthy ones, which is bad for teeth and weight.
Rabbit food lists are not all the same, and it is worth knowing why.
When in doubt, leave it out, and offer more hay.
Wild and garden plants are a big source of trouble. Several common ones are poisonous to rabbits, including lilies, foxglove, ivy, holly, yew, nightshade, buttercup, rhubarb and bulb flowers such as daffodils and tulips. Pick safe greens like dandelion, clover and plantain only from places with no weedkiller, no pesticides and no roadside fumes, and keep houseplants out of reach. If your rabbit eats a plant you can't name, take a photo and call a vet.
Add one new food at a time, in a small amount, and watch the droppings over the next day or two. Soft or smelly droppings mean stop. A rabbit that stops eating or stops producing droppings needs a vet that same day, because a slow gut can turn serious quickly. Rabbits that are very young, pregnant, elderly or ill may need a different diet, so ask your vet for a plan.