An issue that often arises during the sprint retrospective (the most important Scrum event), for new teams is the fact that not enough is known about very new high priority stories to be able to estimate them. This leads to wildly varying estimations, arguments about how or what should be done, committed work not being completed during the iteration and a whole host more problems. It also make the negotiation between the Product owner and the team very 'interesting' during the planning session.
Reasons for these late arrivals could be: