The wood sorrel plant looks like clover and tastes citrusy (it was once used to cure scurvy), but contains oxalic acid, the calcium salt of which is the major component of kidney stones. [link] It can also be a skin irritant and cause many other health problems.
From the NIH: “Many plants contain oxalate, notably rhubarb leaves, dieffenbachia or dumbcane, beets, spinach, mangold, holgeton, sorrel, purslane, dock, greasewood & Russian thistle.” [link]
People eating large amounts of oxalic acid (in rhubarb leaves and wood sorrel soup) have died. The average quantity of oxalic acid per plant varies wildly.
"Rhubarb" CocktailSafe [link]