Recently when mapping froma flat file schema that was created using the Flat File schema wizard I ran across a problem where a test of the map gave an error realated to unexpected data in the source message.
The problem in this case was related to optional, repeating records in the source message/schema.
Eventually I discovered the “Parser Optimization” property, which can be found on the Schema node of the Flat File Schema. This is set to speed by default and cannot handle optional or repeating records. Setting this to “Complexity” allows these types of schemas to be processed and mapping correctly.