[Haskell-beginners] Why is this not a "category"?

[John M. Dlugosz]

on 
https://en.wikibooks.org/w/index.php?title=Haskell/Category_theory&stable=0#Hask.2C_the_Haskell_category

the second exercise in the box (see illustration there) asks
"(Harder.) If we add another morphism to the above example, it fails to be a category. 
Why? Hint: think about associativity of the composition operation."

There are no answers-to-exercises.  Can someone explain to me why adding another function 
with the same type causes the Haskell type system to no longer form the Hask category?

(scratching head)