Suppose you created a workflow, and have used that workflow for some time.
To finish a user task you use the 'Complete User Task' action, where you select the outcome.
Now you change the outcome(s) in of that user task in the workflow.
No errors are shown so you can run your project.
However the 'Complete User Task' action will still use the old outcome and Mendix runtime will fail with a 'Invalid outcome for this user task.' error.
Mendix could check all 'Complete User Task's to see if they use valid outcomes. I would like to find this bug at buildtime instead of at runtime.
We used to have this in earlier versions of Mx workflow. There was however a requirement to be able to use a task page for multiple Workflow User Tasks and had to drop this as part of the consistency validation checking. We will be looking at other ways to inform you as developer at build time.