We found 22 year old bug in COBOL batch job. We kept it.
Currently I am migrating nightly batch jobs from COBOL to Java in core banking system. Some jobs are older than some of my colleagues. Everybody wants to "modernise", which usually means somebody wants microservices. We did not do microservices. We did something boring and it saved us. One job calculates interest accrual for savings accounts. We rewrote it in Java with BigDecimal, unit tests, code review, everything…