Where a task has existing hours added to a billed billing record, it would be useful to see a warning appear if the Revenue Type field is later changed from User Hourly to Not Billable.
I like this idea. I'd like to suggest taking it a step further and allowing system admin's to create Warnings against all of the objects in the system. For example, we have users that will set their project to auto-complete and then attempt to change the status back to Execution, forgetting the initial setting. It would be great if we could build that warning to let them know to check that project setting instead of having to submit a support request to our internal team.