California

https://en.wikipedia.org/wiki/Michael_Cox_(police_officer)

As a plainclothes officer, Cox was mistaken for a suspect and briefly beaten while still in training, and was once purposefully hit by a police vehicle and pinned to a wall. He recovered quickly both of these times and decided not to file complaints.

In 1995, while Cox was working in plainclothes, his car was at the front of a high-speed chase in pursuit of gang members suspected of homicide. The chase involved several cars from the BPD and other departments. Cox continued the chase on foot, but was again mistaken by other officers for a suspect. He was badly beaten by four officers and hospitalized, suffering a serious brain injury. After the officers realized his actual identity (upon one officer noticing the police badge under Cox's jacket), they quickly abandoned him to bleed on the sidewalk. It took Cox six months to recover from many of the physical injuries he incurred.

Cox learned only from newspaper reports that the other officers had failed to report the incident. He has recounted having become upset at efforts by the other officers to hide what had occurred. He began receiving harassing phone calls from other officers even before he had decided whether he would file a complaint. For the next four years, Cox sought acknowledgement of what had taken place, hoping for, at the very least, an apology. In pursuit of this, he ultimately brought a lawsuit against BPD for the incident, which broke years of internal silence within the department on such misconduct, defying what was considered a "blue wall of silence." While seeking to receive some justice for his beating, Cox was rendered a social pariah within the department, and faced threats and harassment from others in the department.

TLDR: undercover cop got beaten by uniformed cops... same could happen to random armed masked men.