Computer systems are way more complex than the simplistic 'one line of code caused the problem' theories we've heard on here. Maybe the final point of failure was one line of code, but that same line of code could have worked perfectly for millions of combinations of events that led to it being executed. It only takes one flawed route to cause a problem.
When analysing failures in computer systems it is usually not too difficult to find where it's failed - the trick is finding why it failed, how did we get to this point with this data on this particular occasion when everything has worked fine for the last umpteen executions.
Think of the holes lining up that people often talk about when analysing aircraft accidents.