Per the User Guide:

"You should not put a restraint or an element between the nozzle node and any specified vessel node. CAESAR II creates the required connectivity from the nozzle flexibility data and any additional stiffnesses between these two nodes erroneously add to the nozzle stiffnesses."

And per CAESAR:
"At nozzle node 4160 you specified a node number that is already used by a restraint, displacement, or hanger. Consider using a different node number for the nozzle or change the node for boundary condition."

Firstly, I'm not certain that it adds "stiffnesses" so much as "adds internal forces to represent the original stiffness." The pipe appears to be moving in the opposite direction.

I'm struggling to conceive of a scenario where a user would want this behavior.

As such, does making this a critical error and not just a warning seem logical?

Being that nozzle flex tends to replace the anchor+cnode design paradigm for many vessel nozzles, it seems this can easily be missed.