This case study describes how a complex data migration was 100% tested using a reconciliation solution. 100% testing means 100% coverage: all non-technical data fields of all records of all tables must be compared before and after the data migration. Custom practice with data migration is to test a representative sample survey, covering 5% or less and only the most critical data fields. Sample surveys, counters and hash codes are insufficient to exclude all possible mismatches and (un)intentional exchanges in the data. 100% coverage is necessary for critical business data, e.g. customer and contract data.