The Baal Shem Tov Hakodesh told the following moshol: There was once a king who was told that if he is humble then he will live forever. He became sooooo humble and started to think to himself "Wow-eeee, not only am I king but I am humble also".
From we see that even a good middah like humility can become skewed.
Added the Holy Ba-al Hatoldos [Rav Yaakov Yosef of Polania, a close talmid of the Besht]: That is what the pasuk means in Megillas Esther "ומרדכי יצא בלבוש מלכות" - Mordechai went out with the garments of kingship. When he WENT OUT, he acted as king. But in his heart, he was humble and realized his nothingness.
One must be careful that his good middos only bring good to himself and the world.
[Heard from the Rebbe Shlita]