[sheepmullet]

That is not really true in practice.

Take for example a GetUserStatistics() call which provides a list of userids and the users last login date.

A client might be using this list to get statistics on system usage.

If you change the codebase to add the concept of a test user and add an isTestUser column to GetUserStatistics() you have broken the contract with your users.

You had an implicit contract based on shared understanding of the data.

Now of course to correctly determine user usage statistics you need to exclude the test users by checking the new column.

[webmaven]

Changing the semantics is entirely besides the point. You can change the semantics of the data without making any schema changes at all!

[sheepmullet]

Isn't it the whole point?

As a consumer it is what I care about.