While doing my Curves presentation at SolidWorks World, I exposed a problem with constraining to projected curves. Here is the entry that I put into the PowerPoint file and the link to the video to show how to do it:

When constraining to projected curves on surfaces or faces, we current (in 2008 SP2) have a problem with overdefining when constraining 3D sketch splines to them. This occurs because projected curves don't meet the ends of the faces that they project to with sufficient accuracy that the 3D constraint-solver needs and it applies both a constraint to the end of the fore-shorten projected curve as well as the edge of the face causing an over-defined condition. If you zoom in very closely on the constraint, you will see this.

It is best to convert them to a 3D sketch curve that will overcome this problem, or you can do the following:

Go ahead and create your 3D sketch spline to the projected curve/s.
When it goes over-constrained, zoom in on each end of the spline until you can notice the mis-match of the projected curve with the edge of the face.
Simply delete the dual constrain to the edge (not to the end of the projected curve), and on either end if necessary, and this will remove the over-defined condition.
You can now successfully add tangent and equal curvature constrains.

To see this Click Here

To play this video you might have to download the Techsmith codec from: http://www.techsmith.com/download/codecs.asp