We have the use case of executing multiple build jobs in sequential and/or parallel order and want to block downstream jobs. We activated option "Block build when upstream project is building" for all downstream build jobs (mostly freestyle-projects), but sometimes downstream jobs are already executed while some (transitive) upstream project is still in progress of being queued (a downstream job could be triggered by more than one parallel executed upstream job that do not finish at the same time). So there might be some kind of race condition in the queue implementation.