My first programming job in 1977 was to convert a set of application systems from one version of COBOL to another version of COBOL for the government of Prince Edward Island. The testing approach was to first run a set of test data through the old system and then run it through the new system to insure that the results from the two matched. If they matched, then the last 3 months of production data were run through both to ensure they, too, matched.
Things went well untill I began to convert the gas tax system that kept records on everyone authorized to purchase gasoline without paying tax. The test data ran fine, but the results using the production data were peculiar. the old and new systems matched, but rather than listing several thousand records, the report listed only 50. I checked the production data file and found it listed only 50 records, not the thousands that were supposed to be there.
the system worked by copying the existing gas tax records file into a new file and making changes in the new file. The old file was then copied to tape backup. There was a bug in the program such that if there were no changes to the file, a new file was created, but no records were copied into it.
I checked the tape backups and found one with the full set of data that was scheduled to be overwritten 3 days after i discovered the problem. The government was only 3 days away from losing all gas tax records.
Question
What might have happend if this bug hadn't been caught and all gas tax records were lost?