Kiel diri "that kind of behavior is unusual"? Tia konduto estas nekutima?
There is actually a subtle difference, even in English. Brown is the usual hair color in my family (kutima). Purple is an unusual hair color (malkutima). Blonde is neither usual nor unusual for my family (nekutima, sed ne malkutima).
The difference is clearer in Esperanto. mal- does not mean "not", it means "the opposite of". malkutima is "the opposite of usual", whereas nekutima is "not usual".