To elaborate on both Webster's and ideath's write up above, cassava is a starchy root, like a potato, and does have poisonous sap. This sap contains amino acid-derived cyanogenic glucosides which, upon eaten raw, the human digestive system will convert to cyanide, which we all know is a Very Bad Thing to eat. It is of dire importance that cassava and anything being made from cassava is thoroughly cooked to remove toxic levels of the glucosides; each root contains enough of the cyanogenic glucosides to be fatal.
A vendor of fried cassava balls in the Philippines learned this lesson the hard way.
On Wednesday, March 9, 2005 it was reported that nearly thirty first and second-grade children died after eating the balls which they had bought (and subsequently distributed to friends) from a vendor near their elementary school (San Jose Elementary School in Mabini). They had suffered severe stomach pains, vomiting, and diarrhea which killed around 30 and critically illed about 35 more.
The vendor of the cassava balls, a woman unidentified in the source of my write up, thought the idea of her tasty snacks killing all those children was absurd. Insisting that absolutely nothing was wrong with her balls, she ate a few to prove her point.
A short time after, she, too, was in critical condition.
Lesson: if they send you running to the stalls, don't eat your own balls.
Source: http://www.cnn.com/2005/WORLD/asiapcf/03/09/mass.poisoning/index.html