If I have a story that's been estimated at 10 story points and we don't manage to complete it in a sprint how do you handle estimates for the sprint in which it's next included (assuming it gets selected for development)? Assuming 80% of development was completed on the story should it be re-estimated at (for argument's sake) 2 story points? Or should the original estimate remain and all 10 story points be burned down when it's finally completed? The former approach seems wrong because a 'big' task now looks like it's small. The latter seems wrong because it surely mis-represents actual velocity in a sprint.
If I write a new story for the remaining work then that also seems like a mis-representation because the original 80% of the work is unaccounted for unless I re-estimate it at 0 points and include it along with the new story for the remaining work and resolve it at the same time as the new 2 story point story is completed.
For those who are interested in the background to this question it's born out of JIRA's default agile board behaviour, which is to return incomplete stories to the top of the backlog. In general we've been re-writing them and/or splitting them into new stories before putting a new estimate against them but I was never sure if this was really the "correct" way of doing it.