The Norse god Balder was the best loved of all the gods. His mother was Frigga, goddess of love and beauty. She loved her son so much that she wanted to make sure no harm would come to him. So she went through the world, securing promises from everything that sprang from the four elements--fire, water, air, and earth--that they would not harm her beloved Balder.
Leave it to Loki, a sly, evil spirit, to find the loophole. The loophole was mistletoe. He made an arrow from its wood. To make the prank even nastier, he took the arrow to Hoder, Balder's brother, who was blind. Guiding Holder's hand, Loki directed the arrow at Balder's heart, and he fell dead.
Frigga's tears became the mistletoe's white berries. In the version of the story with a happy ending, Balder is restored to life, and Frigga is so grateful that she reverses the reputation of the offending plant--making it a symbol of love and promising to bestow a kiss upon anyone who passes under it.
story courtesy of Candlegrove's Ancient Origins