If you sort by incident number, they are no longer sorted in the correct highest to lowest (or lowest to highest) numerical order.

1-In OOB instance, navigate to the incident list
2-create a test record which has number INC999999
3-create another record which has number INC1000012
4-Navigate back to the list view of incident
5-Click on number field to perform the sorting
6-sorting no longer works. 

This is expected behavior and by design in all currently supported releases.

For a workaround adding a zero at the starting of the number(like changing INC999999 to INC0999999) to make sure the sorting works correctly.