I am a Scrum Master for a small development team of 7 developers and 3 testers with 2 week sprints.
The product we are working on is large with a lot of modules. The breakdown of the modules is fairly clean conceptually but their is a lot of interaction between the different modules. And their is a combinatorial explosion in the way they can be used such that it can be a challenge to test all the possible paths through the product.
At the planning meeting we re-estimate tasks left from the old sprint and the developers re-estimate most of them as zero effort as they are "in review" or "in testing". At the end of the sprint most of the stories in the done column have an estimate of zero story points and those that don't have factions of a point.
There are several problems. The team has the habit of under-estimating stories and over-committing in the sprint. The stories snowball as they go through the process. And many team members have other responsibilities and can be pulled off the team at any time during the sprint in a way that is hard to predict. There is also too much work in progress. I have tried banning zero story point estimates but received a rather negative reaction from the team when I suggested this. I have pointed out that review and testing is also work and requires effort for the team but there is resistance to this concept.
I have been pushing for smaller stories and lower commitment over the last few sprints, with some improvement and we have reduced work in progress but we have a long way to go.
With the snowballing effect, a lot more work is usually uncovered in the discovery phase. In review the reviewers have suggestions that sometimes have something to do with the story and sometimes not. In testing the testers find bugs that sometimes have something to do with the story and sometimes not. And everything is added to the old story rather than opening a new one.
I am sure what we are doing is some kind of anti-pattern. I have found reading through PMSEs questions with the scrum tag very helpful, but not all of my questions have been answered by reading though the backlog. So I have jotted down my thoughts in hopes of obtaining some inspiration.