Buttercups, those little yellow flowers that do so much to brighten up meadows and parkland, turn out to be viciously poisonous, along with the whole Ranunculus genus to which they belong. Knowing this puts an interesting spin on the song Build Me Up Buttercup, which is indeed about an attractive-but-toxic would-be lover, but I have no idea if Mike d'Abo and Tony Macaulay had this in mind when they wrote it.
The irritant toxin found in buttercups (a glycoside) causes the mouth to blister, which is probably for the best, since together with their unpleasant taste this helps to discourage anything from eating it. When they are eaten - a danger when animals are forced to live in badly over-grazed fields - they cause similar effects internally, and blistering within the digestive tract is really just no fun at all.
Some species are also pernicious garden weeds. My garden is beset by the aptly-named 'creeping buttercup' (Ranunculus repens), which creeps along sideways by sending out stolons just at the surface, like a sneaky, poisonous strawberry plant. If you try to uproot one, you'll often find that it's connected to another, and then another, and then another. Too much handling of the plant can apparently cause skin irritation and even blistering, although I've never had any problems in my own small-scale weeding efforts.
If someone's chin shows yellow when you hold a buttercup immediately underneath, this is supposed to indicates that they like butter. There may well be a correlation between liking butter and having a shiny chin, but further research is required.