Robert Bates, the volunteer sheriff's deputy who killed an unarmed suspect in Tulsa, Oklahoma, on April 2, says he accidentally fired his handgun when he meant to deploy his stun gun. Bates pled "not guilty" to second-degree manslaughter charges at a court hearing Tuesday. He apologized for killing Eric Harris last week but described his deadly mistake as a common problem in law enforcement, saying: "This has happened a number of times around the country. ... You must believe me, it can happen to anyone."

Some questions and answers about officers who mistakenly fire guns when intending to use stun guns to incapacitate, not kill, suspects:

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HOW OFTEN DOES THIS HAPPEN?

Experts agree this is a real but very rare occurrence that probably happens less than once a year nationwide. A 2012 article published in the monthly law journal of Americans for Effective Law Enforcement documented nine cases in which officers shot suspects with handguns when they said they meant to fire stun guns dating back to 2001. The list included three instances in California and one each in Minnesota, Maryland, Arizona, Washington, Kentucky and Canada. For perspective, Taser International says its stun guns have been deployed more than 2.7 million times in the field.

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WHAT ARE THE CONSEQUENCES FOR THOSE INVOLVED?

Death and injuries for suspects, criminal charges against officers and costly lawsuits for their departments. Including Harris in Tulsa, at least three suspects have been killed in such shootings since 2002 and seven have been wounded. Millions of dollars have been paid in legal settlements. In the previously best-known case involving a California man killed in 2009, an officer was convicted of involuntary manslaughter and sentenced to two years in prison. His department paid $2.8 million to the victim's daughter and her mother. Bates has been charged with second-degree manslaughter in Harris' death.

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WHY DOES THIS HAPPEN?

The way officers carry their weapons, how officers are trained and the stress of dangerous, chaotic situations have been cited. To avoid confusion, officers typically carry their stun guns on their weak sides, away from handguns that are carried on the side of their strong arms. A right-handed officer, for instance, would carry his handgun on his right and his stun gun on his left. In many of the documented cases of confusion, however, the two weapons were holstered near each other on the officers' strong side.

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IN TULSA, BATES SAID HE CARRIED HIS HANDGUN ON HIS RIGHT HIP AND HIS TASER IN A VEST ON HIS CHEST. HAVE THERE BEEN SIMILAR CASES?

Yes. An officer who shot and killed a man in 2009 at a transit station platform near Oakland, California, said he grabbed his handgun from his right side when he meant to grab his Taser X26 from his left side. Cases in 2008 and 2004 in which suspects were accidentally shot and wounded featured similar scenarios, according to the journal article. These instances have prompted a debate over whether officers should be allowed to "cross-draw" their stun guns with their dominant hands by reaching across their bodies at all. Retired Los Angeles Police Department captain Greg Meyer, an expert on the use of force, recommends that officers be required to draw stun guns with their weak hands and then transition them to dominant hands to be fired. "We think that would go a long way toward eliminating some of these confusion incidents that occur under stress," Meyer said.

Some officers do not favor that approach, saying it could lead to other problems handling the weapons. They prefer "cross-drawing" their stun guns, which is allowed under national guidelines published in 2011 by the Police Executive Research Forum.

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IS THERE A NAME FOR THIS SITUATION?

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Bill Lewinski, an expert on police psychology and founder of The Force Science Institute in Mankato, Minnesota, has coined the phrase "slip and capture" errors to describe them. Lewinski, who has testified on behalf of police, has said officers sometimes perform the direct opposite of their intended actions under stress — their actions "slip" and are "captured" by a stronger response. He notes that officers train far more often on drawing and firing their handguns than they do on their stun guns.

Other experts are critical of his theory, calling it junk science and arguing that well-trained officers should not confuse the two weapons.