When you try to delete a Permission Scheme that is associated with a project, the following message will appear:
"If you delete this scheme all associated projects will be associated with the Default Permission Scheme" as per screen-shot:

However, when you try to delete that Permission Scheme, the following error message will appear:
"The scheme cannot be deleted when there are projects associated with it.", as per the following screen-shot:

Both messages seems to convey data inconsistency which is confusing to users.

Steps to Produce:

1. Login to your JIRA instance
2. Create a test permission scheme
3. Go to a project and associate the permission scheme created with that project
4. Now go back to the Permission Schemes and try to delete the permission scheme.