...
Discovered by combination of metrics:
Velocity: Committed vs. Completed, when team does not meet commitments leaving sprint backlog in not completed state by the end of iteration
Scrum Cycle Time, when Max & Average Time increases without any blockers or suspended processes in delivery pipeline (Number of Open Work Stoppages)
Throughput, when Throughput is unstable, although there aren't much changes in team setup/vacations