Consider the DFA, M=({1, 2, 3, 4, 5, 6}, {a, b}, 1, {2, 5}, ), whose  is specified below.I calculated the following and got unmarked states (5,2) and (6,3)(6,3) looks okay but there is something wrong with (5,2).It is not deterministic so where did I go wrong? 
How to minimize the following DFA?
finite automata;nondeterminism
States 2 and 5 are distinguishable. We have $(2, 5)\stackrel{b}{\longrightarrow}(1,5)$ and state 1 is a non-final state while state 5 is final. In addition, as HueHang and Amy have noted, $(6, 3)$ is also distinguishable, so the original DFA is already mimimized.