I have used the Layer Collision Matrix from the Physics Settings to make a particular collider (for clarity's sake, let's call it Colin the Collider :D) collide with nothing but one of the Game Objects. Colin has its own layer, the object it is supposed to collide with has a different layer.
Then, unrelated to this, I have a particle system in the default layer. For some reason, the particles from the system collide with Colin.
How can I prevent this?