Official Description
6.8.3.5 For a case-statement, it is an error if none of the case-constants is equal to the value of the case-index upon entry to the case-statement.
Error Handling
This error is always reported.
NOTE: As an extension Irie Pascal supports the otherwise keyword which matches the case-index if none of the case-constants match the case-index. So technically in that situation this error is not reported.