Gimble Lock
Open the rings.mb file.
Gimble occurs because of the rotation order of an object.
Take a look at the outliner. There are rings which represent each axis of rotation. They are set up with the default rotation order (xyz).
Select the rz ring. Rotate it around the z axis. Notice how all the rings move with the object. Now select the ry ring. Rotate it 90 along the y axis. Notice now how the rx and rz rings are right on top of each other which limits the rotation now. This simulates how gimble lock occurs in maya.
There are a few ways around this. One is to select the object in question and open the attribute editor and change the rotation order. However, this isn't really a fix its a work around which may not work in every shot. There is no way to eliminate it completely. Another way around it is to have another controller, a gimble controller parented above the original controller. Then when the original controller runs into gimble the second controller appears and gives you new rotation controls.