HP 25 Fibonacci sequence in one program step

[C.Ret]

With all the information from each contributor given in this threat, I finally manage to have a Fibonacci Sequence after each [ x ] key pressed :

   

(I cut the top of my printed trace to let other readers seeking for their own solution)
Nice trick, and really specific to RPN; will have hard time making a one key program with the infinite stack of RPL systems.