Scrum has 5 values, one of which is commitment.

We had just had sprint planning, everyone took on as much as they could take on comfortably based on the estimates we had given. We had set some stretch goals as well to make sure we had something to go to if we finished our initial goals.

And we began to work.

a couple weeks in, one of the developers was approached on the side by the functional manager of development with a problem. This issue had to get taken care of, it had to get developed, tested, and finished and out the door immediately.

Not wanting to disappoint this manager, this "Team player" said "Sure, No problem" as we all do when our boss comes into our cube or office. Do we have a choice?

How do you think that this developer, who has committed to exactly how much he thinks he can do in a sprint can take on MORE scope during that sprint? How do you think that 'whatever he did' is going to impact the quality of what work he's doing?

hmm?

Do you think that impacted THAT developer more than it impacted the rest of his team?

What sort of message does this give the product owner who is suppose to be the one single source of backlog items for the team?