I was on the ground floor at George Floyd protests in Seattle and someone mysteriously left a large pile of neatly stacked bricks in the location where police kettled us.
To add: Quebec police infiltrated a peaceful protest while wearing face masks and carrying rocks. In this case the protestors picked up the ruse and kicked them out. Of course they claim they weren't trying to instigate anything.
I saw a video, years ago, where three guys in masks and boots (matching the nearby cops’ boots) were trying to instigate. A protest leader with a white beard was loudly denouncing them and saying they wanted no violence and that they weren’t part of their group. The instigators left, crossing into the crowd of cops in uniform unheeded. Huh, what a coinkidink.