Fishing and saponins.
The "this plant is so toxic it'll even stupefy fish" thing came up on a mailing list I'm on.
Any saponin-rich plant will stupefy fish ... I think it's because soap (or saponins) break the tension of the water, which interfers with the gills.
And that means that saponin-rich plants will do the same to us as soon as we grow gills.
As long as we don't have gills, the only ouchie we get from saponin-rich plants is evacuation, explosive or not, top and/or bottom. (= you vomit and/or get diarrhoea, depending.)
Unless, of course, the plant in question contains other problematic things, in addition to the saponins.
Anyway. Whenever you see "it's poisonous, I tell you, it even stuns fish!" you can snigger knowingly, and, if you wish, educate the spreader of breathless rumours.
Fishing with such plants is illegal wherever it's been practiced. And it's been practiced wherever you find such plants ... mulleins (Verbascum spp.), buckeyes and horse chestnuts (Aesculus spp.), and so on.
Of course, in a survival situation, it's nice to know that this will work, and won't make the fish toxic.
But in a non-survival situation, it's unsporting in the extreme. And illegal.