Have you ever seen a robot spinning, shooting out like a cannonball or rapidly sliding in Gazebo?
Then, my friend, I have to let you know that you messed up the dynamics in your robot’s URDF.
You may now answer, that you don’t even know what dynamics is, and you just want your robot to magically work. Before you go …
… read the tutorial of Gazebo on how to use Meshlab to figure out the inertial parameters of your robot based on the meshes you are using.
So keep calm and specify your inertial parameters, doctors recommend it!