There is a good probability that some stakeholders, the product owner, someone in development team ... or even you, to become unexpectedly unavailable during your current development.
If you believe that your are in-time, you are in fact delayed.
If you are already delayed, you are in fact much more delayed.
If you want to be in time, you need to be ahead.
Look Ahead Modeling: http://agilemodeling.com/essays/modelAhead.htm
(To be continued)