When a thread is created, its SATB queue is always is initialized with the active field set to false. If a thread is created during a concurrent marking phase, this will cause all updates done by that thread not to be enqueued on SATB buffers. This can result in incomplete marking issues.
We have investigating marking issues in G1 as part of 6666145 and this is the one that's causing the most failures. See the Evaluation of that CR for a bit more info.