My lesson in humility came when I was managing a team of 20 and I asked my team to give me feedback on my leadership and management style.  I developed a survey and sent it out to my team asking them to be truthful and honest.  Now I thought I was pretty good in the job.  I had years of experience, great rapport with my team (or so I thought) and we accomplished our goals working well together when the pressure was on.  But the feedback I received was nothing I expected…it hurt bad. I was told I was completely unapproachable, far too direct and I even had a couple people tell me that I hurt their feelings a couple of times.  Needless to say, I was shocked and extremely sad. After a long look at the comments, in the end, I was extremely humbled by this feedback.  I sat down one on one with each team member and learned their leadership style and what I can do to change to help them grow.  And to this day it was one of the most valuable lessons I learned.  After that survey, we were a stronger team, developed stronger relationships and worked together better than ever.  
You might think you are a great leader, but have you asked your team?

Take a look at this article about humble leadership, it really drives it home.