Yikes. The Canadian province of Alberta called in 400 of its police tasers for a study on their effectiveness, and what did they find? Over 10% unleash more voltage than they should.
The good news is that most of those found to be defective were older models—42, specifically, out of 50 found duds. But the majority of tasers in service in Canada and elsewhere likely have a few miles on them (I've seen the Wire, I know how police budgets work!), so this is still not good news.
The study is related to an ongoing investigation of the death of a man at a Vancouver airport in 2007, who was tased 5 times by police. Which has to be a rough way to go. So if you're in a situation where a tasing seems likely, make sure you check the model number in the cop's hand before you get hit with the juice! Yikes I say!