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I've found 2 reasons (empirically) for this problem so far. After resolving both issues, I have not seen any problems (tested under ROS Groovy + Gazebo 1.2.5)

1. All my meshes were made in sketchup or blender, and I managed to invert a few faces and the mesh did not close properly. This affects Gazebo in weird ways. Some of these errors may not be visible in sketchup/blender, but were visible in meshlab.

   • Output of a "good" mesh in meshlab. All collisions in this mesh were fine, irrespective of whether the colliding surfaces had surface properties defined or not.
   • Output of a "bad" mesh. the top part is not completely closed. This mesh would cause problems in the middle of the room - which is empty.
   • Resolution: It is easier making the meshes in sketchup while making sure none of the faces were inverted (check in meshlab). Also DO NOT export 2 sided faces - the robot was unable to move inside of the room.

2. The erratic differential drive plugin changes the location of the model in the simulator instead of simply imparting the appropriate velocity to the wheel. Consequently the robot would overlap a tiny amount with a wall and bad things happen.

   • Resolution: This problem went away by improving the plugin to only impart joint velocities (see here)

Somebody familiar with Gazebo might be able to provide a better explanation on why an incomplete mesh failed.