Working on API part of current project, we were prepared with the API specs. The application that we are writing is supposed to replace the old and expensive on-the-shelf application that they bought licenses from overseas vendor years ago. In short, we writes an application that provides same functionalities at cheaper price for our client.

After it is all done, the client mentioned some of the API calls return wrong/incorrect response.

Furios, went to check the specific API call and confirmed that there's nothing wrong with it - all according to specs.

It turned out that the client didn't check against the specs, but with their current application instead.

That application didn't meet the specs they provide 100% (not broken functionalities but rather it was full with bugs here and there) and apparently the client have gotten used to it.

Now, we are writing our application to provide the same buggy API responses.... because the client doesn't want to introduce change to their clients.

I am writing to provide an empty json array instead of actual data....wtf