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The object wiggles out of the robot's grasp because ODE's simulation of contacts is not very realistic. You can add John Hsu's grasp hack to the object to make it much harder to drop the object--when both fingertips have contacted the object for some number of seconds, it records the relative pose between the object and the gripper and keeps warping the object back there until the gripper opens and lets go. (Sometimes the object still falls if you move around too fast or hit the world with it, but that's actually more realistic than otherwise.)

Look at cup.urdf in manipulation_worlds/urdf, in stack pr2_object_manipulation, for an example of how to add the grasp hack to an object.