Bullet Physics Pitfalls
Bullet physics is not without its problems. Unfortunately, many of those are outside of the control of the jMonkeyEngine Core Team and thus cannot be fixed.
Sweep Test Issues
When using link:https://javadoc.jmonkeyengine.org/v3.4.1-stable/com/jme3/bullet/PhysicsSpace.html#sweepTest(com.jme3.bullet.collision.shapes.CollisionShape, com.jme3.math.Transform, com.jme3.math.Transform)[PhysicsSpace.sweepTest()], ensure that the distance between the transforms is at least 0.4wu or greater.
Note that the sweep will not detect collisions if it done inside of a collision shape. It must be on the edge of a collision shape to detect any collisions.
Ghost Control AABB Collision only
As the javadoc for PhysicsObjectControl says, the ghost object collision detection uses AABB (Axis-aligned bounding box) collision only, regardless of the collision shape it has been assigned.
Workaround:+ Please use PhysicsSpace.sweepTest() instead, or kinematic physics objects with collision listeners.
Rigid bodies fall through ground
This usually happens if the ground physics object has large triangles or consists of a large BoxCollisionShape.
For meshes with large triangles - Subdivide the mesh in a model editor such as Blender.
For large boxes - separate into smaller boxes or use a MeshCollisionShape for terrain instead of BoxCollisionShape.