Green Tomatoes, Green (raw Potatoes)
These foods are members of the Solanaceae family of plants, which includes the Deadly Nightshade, and contain a bitter, poisonous alkaloid called Glycoalkaloid Solanine, which can cause violent lower gastrointestinal symptoms. The leaves and stems are particularly toxic. (Tomatoes in pet foods are ripe, and should cause no concern)
this is just one source, there are several more that state similar warnings.
I, too, once had a cat who enjoyed eating potato skins from the compost bucket. I didn't know at the time that they were bad for her, but I did discourage it anyway, because I don't give my cats people food. Cats bodies are very different than ours, and you just never know what's going to be bad for them.