I was always under impression that Sprint Backlog is part of Product Backlog -> it's impossible to add anything into Sprint Backlog without affecting Product Backlog

However, according to one of the comments to [this post][1], these backlogs "are completely different artifacts"

We are using an electronic Scrum board; would it be possible to add things like items from our Retrospective into Sprint backlog and avoid messing up Product backlog?

  [1]: http://pm.stackexchange.com/questions/7980/how-should-we-implement-items-from-sprint-retrospectives/8013#8013