It happens, but very,very rarely and almost never publicly.
Instead, they close ranks and state that everything police do, no matter how outrageous or criminal, is justified.
Then they burn through millions of dollars every year having their criminal actions defended in court free to them at tax payer expense and when multi-million dollar judgements against them come in, that gets paid by the taxpayer too.
Here's a man who was Chief of Police of Dallas admitting that many police don't know how to behave and should not be police. A common sense observation, but one normally not allowed on television.