When a team commits to completing a set of stories for a sprint, it is up to the Product Owners to constantly be viewing the Burn Down or Burn Up charts very early on in the sprint and check the velocity. If the velocity shows that the team will not be on track to finish all the stories that have been put in the sprint, it is extremely important to pull stories early in the sprint and not late. The reason is because when a team feels they have more work then they can handle they will rush to try and finish it all for the end of the sprint. This is bad because this is where many bugs get created. Pulling stories early on is far better for the team. It is better to under shoot and add extra items at the end of the sprint, then to over shoot and not complete items or have to pull items at the end of the sprint.
I thought this discussion was interesting and made a lot of sense.