Ever got this message in the execution of the workflow: Work item could not be created. Insufficient rights for user %1?
One of the possible reasons for receiving this is that one of the menu items specified on the approval or task elements for this step in the workflow is having it's configuration key disabled. You might look on this for starters.
The stack trace that will lead you to this is: SysWorkflow\save -> SysWorkflowWorkItems\createWorkItems -> SysWorkflowWorkItems\create -> SysWorkflowDocument\assertPermissions -> SysWorkflowDocument\assert -> SysWorkflowDocument\assertAsUser.