Working as a coach I sometimes hear the complaint that interruptions and additional tasks cause significant pain to a development team.
Often people are asked to do things that were unknown at the start of a sprint/iteration but appear to be urgent/important when the sprint is under way. Since these activities are unplanned and unbudgeted taking on these tasks can put the sprint goals in jeopardy. It may be that the requests seem innocuous - an hour here or there, what harm can it cause?
Individually these additional tasks may not seem significant but these interruptions add up and can start taking over. They also establish a pattern for the future and people get used to asking for more and more of your time.
Switching tasks and not being able to complete one task before being pulled off onto another one should not be underestimated. It is not easy to pick up some development work even after a break of an hour or two!
Keep a list of things that you have done during a sprint that can be used to adjust capacity in the next one. Make sure that the Scrum Master or Iteration Manager is aware of these interruptions (they should be consulted anyway). Make sure that these additional tasks really are more important that the current goals!!