What is a dead-end spline? Look at the image on the right. Ideally, every CP in a well-formed mesh should have two splines that pass through it. It's okay for a CP at the edge of a mesh to have only one spline passing through it and another that ends at the CP, but it is never a good idea to have more than two -- unless you really want massive creasing. Having more than two splines passing through a CP practically guarantees creasing. Especially if, as here, two splines pass through a CP and a third just butts up against it. That's a dead-end spline. There are two in the image on the right (colored green, with arrows pointing to them), and 34 in the mesh above.
As we will see, in addition to creasing, dead-end splines can cause a more serious problem -- they can make defining 5-point patches impossible.