I have a (now simplified) model of a rower in a boat that keeps crashing or is very unstable. The code can be found in my github.
See the model in the file boot.xacro. There also is a boot.sdf that is generated from that, with only the ros_plugin removed so it can directly be run in gazebo.
Starting this model most of the time shows a wildly unstable behaviour, see this video.
If I change the shoulder joint to have 3 instead of 2 revolute joints, then the model crashes immediately, see this video.
I cannot find anything wrong with the model, all masses, inertias etc. are sane, so I only can conclude there is a bug here.