It's easy to cry 'bug' when the truth is that you've got a complex system and sometimes it takes a while to get all the components to co-exist peacefully.
As soon as we started programming, we found out to our surprise that it wasn't as easy to get programs right as we had thought. Debugging had to be discovered. I can remember the exact instant when I realized that a large part of my life from then on was going to be spent in finding mistakes in my own programs.