Remember, back in grade school, when we used to get snow days? That is, when there was so much snow that they announced over the radio that school was closed for the day and — even better — we got to spend all day playing in the snow? Remember that year when we had so many snow days that we had to stay in school a couple extra days at the beginning of summer to make up for them while kids from other schools got to start vacation? That kind of sucked.
On agile teams — even experienced ones — I often seen people take snow days. Sometimes the estimates were simply too conservative, and sometimes all the remaining development tasks get blocked by external dependencies. Either way, team members find themselves with no open tasks to complete days before the end of the iteration and, instead of going to the team, the project manager, the tech lead, or the team customer to ask what they should work on next, they take a couple snow days. Of course they come to work and of course they do work, but they often spend the snow days on interesting but low priority tasks.