Const. Andrew Vanderburgh had only been on the job for about a year when he responded to a fateful call in November of 2009.
In court on Monday, Vanderburgh testified that he had received a call about a driver swerving all over the road.
The licence plate led him to a Toronto apartment where Breton Berthiaume, a Halton police officer, was just arriving home.
Vanderburgh told the court that Berthiaume had a "strong odour of alcohol," had slurred speech and was fumbling with his documents.
The Toronto officer saw a badge, but arrested Berthiaume nonetheless.
When Vanderburgh then brought the man to a police station, he was allegedly called a "rat" and was harassed and berated by other officers.
And when Vanderburgh drove home that night, another officer pulled him over and gave him a ticket for running a red light. That traffic ticket was later thrown out and the officers involved in the harassment were found guilty of misconduct.
Berthiaume has pleaded not guilty to a pair of impaired driving-related charges. His lawyer argues that Vanderburgh did not follow proper procedure when arresting Berthiaume, and that his notes were incomplete.
Click on the video above to see a full report from the CBC's Jeff Semple.