There is a temptation to shift the duration of your sprint when it isn’t going as expected.
Maybe more customer issues came in, maybe your estimates were off, or perhaps most of the team was out sick with the stomach flu, whatever the reason, the temptation to add a few days or weeks is always there so we can say that we finished work “within the sprint”.
The problem with this approach is that your Sprint never ends, and once you do it once, it gets easier to do it again and again and again until you are on week 5 of a 6-week sprint.
The beauty of a Sprint isn’t the work done during it (although that is very important), it’s that you do the reset, the re-plan, the retrospective on what did and didn’t work, and adjust going forward. If you are pushing out those adjustment steps, you are pushing out the opportunity to reset your team, adjust where they are going and reset your team every 2 to 3 weeks.
And when you are headed down in development, you need this time in order to help them stay on point.