It is especially easy to notice when you observe new teams. Most new teams do not fit into planning time box. Not experienced scrum master and product owner will make all effort to continue planning and fit maximum into upcoming sprint. If you notice such behavior - stop it right now and start working on those things that team managed to plan. Based on experience, if team doesn’t make it into a time frame it means that they are not capable to understand and commit on this at the moment. Leave it for now and let the team to learn!
All time boxes are need to understand your capacity, that’s it.
p.s. picture is taken from here - http://blog.3back.com/tag/time-box 🙂