As a web craftsman, how much effort should you put into fixing past mistakes? obviously glaring errors should be fixed, but what about when you learn something new?

I recently had a case study published on SitePoint. I no longer work for the Ministry of Justice where I did the work that is the subject of the study. However, responses from readers have shown me how to fix problems that I was not aware of at the time, and fixes for problems that I could not solve.

Of course, I have learnt from this and will not make the same mistakes. The manager that requested the work is happy, but I still feel oliged to fix them. Is it different because the work was for an internal customer, rather than a paying client? Would I feel different if it had been a frelance job?