Do you know Kernighan's Law? 'Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it.'
I actually did not know Kernighan's Law ... but once I heard I intuitively agreed with it.
What's really hard is debugging code from someone who is way smarter than you and writes clever code. That takes a great deal of persistence.