A lot of things can go wrong during a Scrum sprint. In his blog post, Ilan Goldstein makes a list of some common issues in Scrum sprints and how you should deal with them. He recommends to gather as much data about these speed humps as possible so that you can inspect and adapt your processes and start bridging the gap between theoretical planning and reality.
The sprint issues covered in this blog post are:
* Team overcommits – how do you roll user stories (and other product backlog items) into the next sprint?
* Team undercommits – should you add new user stories mid-sprint?
* Your task estimates were incorrect – should you adjust them and reset the burndown chart?
* External impediments – how should these be reflected in the burndown chart and velocity?
* Product Owner changes – should you allow them to remove, add or significantly modify the sprint’s user stories?