I used inheritance rather than composition when writing a wrapper to DataTable (before extension methods existed) in C#. I fixed it later for future companies,

I used inheritance rather than composition when writing a wrapper to DataTable (before extension methods existed) in C#. I fixed it later for future companies, but it ran in about ten times the time it should have taken and it killed the product. A little while later I offered to come back to the company to fix the mistake. it would only have taken 2 to 3 hours, but by then the product was dead.