Perhaps you've heard of the phenomenon of kids getting hit by a baseball in the chest and dying? It doesn't happen often. Apparently there's a one or two thousandth of a second window during the cycle of a heartbeat where such an impact can disrupt the electical impulse and essentially stop the heart. This isn't urban legend. Here is a source and a quote:

As described in a previous paper [12], the Consumer Product Safety Commission conducted a study on sports injuries in children between S and 14 years of age. The study found more baseball related fatalities in the 5- to l-year-old age group than for any other sport [2, 12]. In a follow-up study, 51 baseball-related deaths of children were documented [9]. The most frequent type, 21 cases in total, involved impact of a ball to the child's chest. Of those fatalities, 11 occurred during organized games and the remainder in unorganized recreational play. The Consumer Product Safety Commission has since reported another 11 deaths in children secondary to chest impact from a ball between 1983 to 1990 [10].

Now it should come as no surprise that deliberately running a 50,000 volt shock to the body might, just might, disrupt those heartbeats as well. I do understand the reasoning behind using tasers. They are undoubtedly less lethal than guns, probably less chance of wounding a bystander, etc. But this supposes a direct comparison to gun use, i.e., that every use of a taser is a situation where a gun is the only alternative means of force. Somehow, I don't think so. This is from The Trouble wth Taser by Anne-Marie Cusac at the Progressive mag site:

High-powered tasers are the new fad in law enforcement. They are becoming ever more prevalent even as their safety is increasingly in question. The proliferation of tasers in police departments across the country has led to unconventional uses. Among those hit by tasers are elderly people, children as young as one year old, people apparently suffering diabetic shock and epileptic seizures, people already bound in restraints, and hospital mental patients. Police used tasers against protesters at the 2003 Miami Free Trade Area of the Americas demonstration and against rowdy fans at the 2005 Fiesta Bowl. School systems are employing the weapons, with some officers carrying tasers even in elementary schools.

But doctors, reporters, and human rights groups have raised questions about the safety of the devices, which shoot two barbs designed to pierce the skin. The barbs are at the end of electrical wires carrying 50,000 volts. Last summer, The New York Times reported that at least fifty people had died within a short time after being hit with a taser. By November, when Amnesty International released its own report, that number had risen to more than seventy.

In February, Chicago police used the device against a fourteen-year-old boy, who went into cardiac arrest but survived, and a fifty-four-year-old man, who died. The Chicago Police Department, which had recently purchased 100 of the devices, decided not to distribute them until it had investigated the incidents.