It's difficult to criticize either party without more details however as a developer (especially one contracted to do a job) I feel some ownership over my work - not from the viewpoint of expecting additional compensation or the ability to use that work since it was a contract-for-hire but some responsibility and desire to see my work used productively.

There are several things I think you should have done -

1) Determine the root cause of the failures - is there something in your solution that you should have been aware of that is now causing the issue?

2) Build into your contracts a guarantee period and/or a maintenance fee retainer that creates a fair and equitable situation in the event that "something breaks when something else changes". This should be easy to get accepted if the product you are creating generates real value for the customer, value that would be at risk should the product fail.

3) Provide enough documentation so that mid-level programmers understand the design, functional components, possible exceptions and things deliberately not addressed by your solution.

4) Look at the situation as another opportunity to show this client what a fantastic developer (and business person) you are rather than a "complaint" from a customer. Customers/users report issues - they don't complain. You have to use your tact and common sense to turn "this program sucks" into an actionable plan to make it suck less.