http://www.swampyacresfarm.com/wordpress/poison-volunteer-squash-warned/
Happened to see this discussion and thought this first-hand account of someone supposedly being poisoned by a squash would be appropriate to add here.
I am trying to find out if it’s ok to eat decorative squash (fleshy, not woody like a gourd).
The thread is nine years old. Everyone died from squash poisoning.
If only they had known not to eat the bitter stuff!
Threads are forever.
I know this is an old thread, but yup, I’d heard of this.
It’s actually pretty hard to be poisoned, as curcurbitacin, the problem chemical, is so inedible bitter that any fruit with a high enough concentration to be risky is going to taste so foul you’d need to force yourself to eat it. Accidental crosses may produce high levels in fruit, but sometimes otherwise normal named variety plants will produce a freakishly high concentration of it, sometimes in just one fruit, probably in response to stress (never heard of it happening in commercial plants, but I know home growers who’ve had this happen). If you’re not sure, taste it before making a whole batch of squash curry or something.
It’s pretty rare, and normally just makes you feel awful rather than being truly dangerous.
Pretty much the worse thing that could happen with volunteer squash is that the genes crossed and the resulting fruit won’t taste good. I’ve heard of people planting saved seeds that they thought were, for instance, pumpkins, and got some kind of strange cucumber/zucchini/pumpkin cross that looked interesting and tasted even more interesting, and not in a good way. In at least one case, they decided to feed these to their backyard poultry (this, BTW, is what’s usually done with giant pumpkins, which are technically edible but don’t taste good either) and those chickens and ducks made some very yummy eggs. 
I almost ate some poisonous volunteer squash once. But at the last minute my wife stopped me. It was a marrow escape.
Zucchini (aka courgettes) are particularly susceptible to reversion to wild phenotype, and can even cross pollinate with poisonous ornamental squashes, yet some people get it in their heads that commercial seeds = BAD and saved seeds = GOOD. A German man died after eating a stew made from his neighbor’s heirloom zucchini. Thinking he was eating ‘health food’, he ignored the incredibly bitter poison and finished the whole bowl, which ended up killing him.
We have standards here, you know!
They’re low, but we have them!
Okay, you’re a keeper.
But you will have to go.