Read this Fat-finger incident. And reflect on what went wrong?

When dealers are in a hurry or simply get tired, they can push the wrong button, turning $3million trade into $30million or $300 million trade. On the morning of December 8, 2005, an unlucky clerk for the Japanese firm Mizuho Securities discovered this. Instead of entering an order to sell one single share of J-com, a small Japanese recruiting firm, at a price of 610,000 yen (about $5200), the clerk placed an order to sell 610,000 shares at 1 yen (less than $0.1) apiece. This eventually cost Mizuho $340 million. "