The physical simulation engine can be quite sensitive to the configuration of bodies in a simulation. This can lead to performance problems, instabilities and unexpected results. Because of the number of factors involved, it can be difficult to track down the exact reason for instabilities or unexpected results.
Below are some things to watch out for when dealing with physical simulation.
Instabilities may arise when dealing with very large (heavy) and very small (light) objects
Exerting very large joint pressures or velocities may cause unrealistic behaviors.
Beware of joints which force invalid or conflicting conformations on your objects.
Avoid forcing deep collisions between objects with either joints or manual movement.
Use of redundant or unnecessary joints will cause instabilities and performance problems.