Ever since I was a child, I've heard women in my country talk about removing the corncockle from the grain before they cooked wheat porridge. In Romania we eat lots of wheat or barley, which we use in a special dessert called "coliva" for certain religious feasts. Barley is better than wheat for this dish. Barley, as well as wheat, inevitably contains some corncockle seeds. Not many, but if any of them are cooked with the wheat or barley, the porridge will have a bitter taste. That's why I thoroughly remove the corncockle seeds from the barley when I prepare coliva.
I've learned to recognize corncockle seeds, but I didn't know anything about the plant itself, until I saw this beautiful purple flower growing in a field near my house. Like any other gardener and plant addict, I wanted to identify it, so I did my research. First I searched Romanian wildflowers websites and found its latin name, Agrostemma githago. The Romanian common name "neghina" was a real surprise - it was the plant whose seeds needed to be removed from the grain! A simple search on Dave's Garden Plant Files showed me its English common name was, corncockle.
Corncockle is a weed native to Europe, which grows on the fields, wasteland or roadsides. Its Latin name, Agrostemma githago, refers to the two toxic saponins, githagin and agrostemmic acid, which the plant contains. Corncockle was a common weed of the wheat fields in the past, but not any more, starting with the 20th century. Due to increased use of herbicides and mechanised farming, the plant is no longer a common weed of the crop fields. Moreover, the wheat is now sown in the fall (winter wheat) and harvested before the corncockle sets seeds. Yet, it is likely that most of the sown wheat contains some corncockle seeds. 
The plant is slender and beautiful. The leaves are narrowly lanceolate, pale green. Each branch is errect and tipped with a beautiful pink flower. Sepals are longer than petals and narrow, forming a tube at the base of the flower. Stalk, leaves and sepals are covered with fine hairs. Seeds are dark brown and grow inside capsules formed by the joined sepals at the base of the flower. Bumblebees seem to like their nectar!
All parts of the plant are poisonous because of the saponins it contains. These have a hemolytic effect if ingested. Corncockle seeds can cause diarrhea, nausea, vomit, respiratory depression and even death. The plant has the same effect on livestock and poultry. If corncockle seeds remain inside the wheat when ground, the resulting flour will be darker and will have a sour taste. However, the bread made with that flour won't be poisonous because heat partially destroys the saponins' toxicity.
I always check the barley before I cook, and remove any corncockle seeds. Now that I know how serous the side effects are, I will be sure to be even more careful when doing this task! I'll just put on my glasses and check every grain, so I can find the tiny corncockle seeds and remove them.
Corncockle is a beautiful plant, but I was a little surprised to see how many websites list corncockle seeds in their catalogues. I have to agree, it would make a beautiful display in one's garden. And it would be such a waste of this beautiful flower if it were to vanish, so better in people's gardens than in the wheat fields. I would be careful to deadhead the plants if I grew it in my flower garden. Making as many seeds as it does, you wouldn't want it to spread out of your garden and get into nearby wheat fields, then start the cycle all over again. Fortunately, modern agricultural methods won't allow corncockle to spread. So, if you like the flower, go for it, but don't eat the seeds!
 - http://en.wikipedia.org/wiki/Common_Corncockle
 - http://www.ansci.cornell.edu/plants/toxicagents/saponin.html
 - http://www.moraritsipanificatie.eu/2008/11/principalele-seminte-toxice-de-buruieni.html