'Resource Overloaded' Exception Message Not Appearing in Unconstrained Plan

Though Horizontal plan shows required hours more than available hours for a resource , resource overload exception message is not appearing in Unconstrained Plan

In the User-guide it is mentioned as below

" This exception message appears when, in a planning time bucket, both:

• The resource required capacity is more than the resource available capacity
• The load ratio is more than the exception set Over-utilization  "

In our case we haven't defined exception Set . It is mandatory to define exception set to get 'Resource Overload' exception message . (My understanding was exception set is optional and may not be required unless we need to filter unwanted exception messages)