I have been searching the forums for an answer to this and am coming up with nothing.
I have a cube that I'm using as a placeholder for an item that can be picked up. It's box collider has Is Trigger checked, but when I walk into it I'm stopped and OnTriggerEnter isn't called.
This was working fine last night, until I made an Empty GameObject and put the functionality into it, then made the Mesh a child.
I've since deleted everything, started over, and am back to the original, simple cube with a collider and my script attached. I also deleted and re added the FPS controller. I've tried walking into the cube with Is Trigger checked and unchecked, and checking and unchecking it during play. The only thing that makes a difference is if I remove the Mesh Filter during play, suddenly it works, I can walk through it, the trigger activates and the cube is picked up. Of course, doing that outside of play deletes the mesh.