>There must be thousands, possibly over ten thousand of these incidents every year.

It's a shame we can't have any reliable statistics based insight into exactly how often it happens because of the widespread policy amongst US police departments of not collecting and/or publicizing such data. The Obama DOJ was attempting to correct this trend (in a collaborative manner, which was the only available avenue) with many such departments, but cancelling that policy initiative was literally Jeff Sessions's first policy action as AG.