This issue comes primarily from the specs2 project (issue 62).
I'm thinking that this could be a sbt issue with some kind of concurrency problem when incrementing the counts here.
IMPORTANT This project has moved to https://github.com/sbt/sbt. This issue still exists to avoid dead links, but GitHub limitations prevent the original issue from being correctly preserved here. Please see sbt/sbt#372 for the full issue and to comment.