They are very bitter before they are ripe. The seeds are of the devil. Otherwise, they are fine.
Just hire a carpenter.
Very sour, but crab apple jelly is delicious.
Some types of persimmons are inedible until they’re so ripe that they’re mushy. The problem is tannins, which exist in many plant foods. Tannins bind to proteins, and in large amounts make your mouth get dry and puckery. Very unpleasant.
The two most common types of persimmons in the U.S. are Hachiya and Fuyu. The former has the tannin problem, and is recognizable by its pointy end. The Fuyu can be eaten while still firm, and has a round or flat end.
You have this the opposite way around… grapefruit juice can make medications too effective (by inhibiting their metabolism). Earl Grey tea works the same way, and if you ever happen to eat a bergamot orange, it’s the same stuff.
Natto (Japanese fermented soybeans) has anticoagulants that will prevent blood clotting if you eat too much of it. I might be the only person actually at risk of this.
Eh, I don’t have any objection to lima beans, but they do taste different from most other beans, to me.
Like grapes and chocolate are bad for dogs, onions are bad for cats.
Okay, who’s gonna go and try this? 
Hmmm…I’m gonna have to experiment and see what refried lima beans taste like and see how different it is from pintos. (Idea being to mash the crap out of them so the shape and size is not a factor in my perceptions). I have memories of not particularly liking them as a kid, but then a few months ago I made a Jamaican oxtail stew that called for lima beans (or butter beans, same thing as far as I know, but I used limas) and I was surprised at how “normal” they tasted to me. It was just like a big, creamy bean.
Just unpleasant.
A local grocery store here used to sell crab apple jelly, which was actually quite good (ton of sugar in it) and crab apples have long been used for cooking.
Domestic apples are crab apples that have been bred for more desirable traits and fewer undesirable ones.
Puffer fish?
Swallowed fish bones, if they get stuck in your throat, may have to be removed surgically, and can cause tissue damage that will result in lifetime discomfort.
All three are bad for both. I would be surprised if ferrets didn’t also have the same problem among common pets - since the issues are common among both cats and dogs, they’re probable true for carnivorans in general.
The chocolate hazard to pets is overrated: On a per-weight basis, theobromine is only about twice as toxic to dogs and cats as it is to humans. The main difference are just that they’re smaller, and that they (especially dogs) show less restraint in what they’ll eat in one sitting.
Fugu. See above.
Maybe related to the Lima beans already mentioned, but Fava beans can cause anemia in people who lack/are deficient in a specific enzyme (“Favism”).
Not dangerous but certainly strange: purple cabbage is a natural pH indicator. Boil or steam some, and collect the beautiful purple water. Purple is neutral. Add some vinegar: pink! Add some baking soda: blue! I always try to titrate back to neutral, but it’s tough. I have impressed many kids with this.
The same is true of turmeric. It’s yellow in acid or neutral conditions, but turns red in basic conditions.
No kidding? I’ll have to check that out with turmeric. I knew the cabbage thing, but we also had some kind of violets in the back yard that, as I kid, I discovered also acted like a pH indicator. I’m not exactly sure how I stumbled on it. Maybe I just extrapolated that since purple cabbage made for a pH indicator, that purple things were all similarly reactive? At any rate, the purple flowers (I don’t know if they were properly violets) worked in the same way. I would crush them and steep them, and the solution would turn one color when acidic and another when basic. I can’t remember the colors, though. Was it blue one way and red the other? Or maybe blue and green.