I haven't really maintained this software (as in doing on-going support) but I've added a few minor features like image and layout changes to an ancient VB 5 Windows application mostly by using a hex editor, changing the right assets and doing some rather interesting work-arounds in the underlying Access database.

It was both fun and scary. The software was a custom-built CRM / ERP system that must have cost a small fortune to develop. The company that developed the software has gone bankrupt quite some ago. I asked the former developers about the source code but they've simply lost it in all those years since 1997 or so when the application was originally developed.

So, here we have a company that's paid a large amount of money for a software - which admittedly is working quite well till this day - they now can't easily change anymore. Even something as simple as changing their postal address or adding additional branch offices takes considerable effort.

This should serve as a cautionary tale to always require some sort of source code access (preferably open source but some sort of escrow for the case the original creator goes under might work as well) to software that's pivotal to your business.