This question raises a common problem, that is trying to determine what part of SDF is causing a simulation problem.
We need more warning and error messages.
Also make it possible to watch and plot values of models and the world.
On a related note:
It could be helpful to keep the display state in the model property list when using tools (ie. move, rotate).
Currently, if I want to watch the pose values in the Property tree, I select an object with the tool I want to use and expand the pose variable to see (x,y,z,roll,pitch,yaw). Then I click again on the object to move/rotate it and the Property list refreshes with pose collapsed so I can't watch it.
This is related to the idea of a "watch window".
Adding to Steven's note:
This behaviour would also be good when switching between objects. i.e. when you want to compare the properties of two objects it would be good if the property tree stays expanded, showing the values of the newly selected object.