Apparently, they occasionally do, and don't even get a slap on the wrist at the end of it all:
In January 2007, a SWAT team in Lima, Ohio, shot and killed Tarika Wilson, a 26-year-old mother, during a drug raid at the home of her boyfriend, Anthony Terry. When the unarmed Wilson was shot, she was kneeling on the ground, complying with police orders. She was holding her 1-year-old son, Sincere, who was also shot, losing his left hand. A subsequent investigation revealed that Officer Joseph Chavalia heard another officer shooting Terry’s two dogs, mistook the noise for hostile gunfire, panicked, and fired blindly into the room where Wilson was kneeling. Chavalia was charged with involuntary manslaughter, but acquitted.
A Denver Post investigation found that in 80 percent of no-knock raids conducted in Denver in 1999, police assertions that there would be weapons in the targeted home turned out to be wrong. A separate investigation by the Rocky Mountain News found that of the 146 no-knock warrants served in Denver in 1999, just 49 resulted in criminal charges, and only two resulted in prison time. Media investigations produced similar results after high-profile mistaken raids in New York City in 2003, in Atlanta in 2007, and in Orlando and Palm Beach, Florida, in 1998. When the results of the Denver investigation were revealed, former prosecutor Craig Silverman said, “When you have that violent intrusion on people’s homes with so little results, you have to ask why.”
Lima police apparently aren’t as concerned. When told of the Lima News investigation, police spokesman Kevin Martin said, “That means 68 percent of the time, we’re getting guns or drugs off the street. We’re not looking at it as a win-loss record like a football team does.”
And Chavalia is back on the job as well.