I have seen many rewrites for financial institutions in my life; one of the biggest I had a good insight look at during the entire project was a Tandem Nonstop

I have seen many rewrites for financial institutions in my life; one of the biggest I had a good insight look at during the entire project was a Tandem Nonstop Cobol rewrite to Java; the famous 10s of millions of lines to nice clean Java code. Many of them never work; this one was, if I remember correctly, upwards of 50m Euros and was thrown away because it did not work in the end. I think the Tandems are still running.