Steps to reproduce :
Create a workflow.
Add a stage with a concurrency of 0.

Expected :

• Either the concurrency value is rejected on saving,
• Or the stage actually has no concurrency constraints (this what I expected when I hit this).

Actual :
The workflow is blocked with the message "Waiting for builds []".
When it reaches this point, the workflow cannot be aborted, ever.
The log states "Aborted by whoever" each time someone tries to abort the build, but the workflow is still blocked.
It is still blocked even after a Jenkins restart.