System: MacOS 10.12
Blender Version: 2.78a
Issue: UV's are able to be scaled beyond bounds in 'constrain to bounds mode' when pivot point is set to 2D cursor, and positioned on an edge. (possibly other scenarios as well- however that is what I have experimented with)
This issue can also be reproduced on the x axis as well, it seems to be dependent on where the 2D cursor is placed within the UV workspace.