We use the "Sub-Task Blocking Condition" to block transition of a parent issue to Done until all Sub-Tasks have been marked Done, Cancelled, etc.

When this condition is triggered due to incomplete Sub-Tasks, the blocked "Done" transition button simply does not appear on-screen. To users who have forgotten or are unfamiliar with our process (sub-tasks must be completed first...), this looks more like a JIRA error than an intentional consequence of the users actions. 

Instead of hiding the blocked transition, please show the blocked transition as disabled on the screen. When the user hovers over the button or tries to click it, a tool-tip type message can display showing the reason the transition is blocked. ie, "All Sub-Tasks must be [selected Statuses] to transition this issue to [destination Status]."