Personally, I think the best thing is education. Now more than ever (in this agile / scrum world) devs are making small, quick changes to existing codebases. These small changes add up, and what was once a great, clean codebase is now riddled with hacks and mis-documented code.

I think educating the developer / team on the existing codebase first would be the right step. Spend a day just going through the code. Learn how it connects, learn the full scope of the project. After this, make your changes in a way that fit into the existing code.

I've had too many offshore developers re-imnplement existing functionality, like reading data from an excel spreadsheet, simply because they did not look through the code that was already there. It cost them 2 hours of work to add another library and implement it, when the code they already needed was a simple using statement away.

In theory, this is why good orgs have senior devs: they know the codebases already. Furthermore, they should be reviewing these changesets to call out devs who mis-document or scatter about the code. But senior devs are people too, and they miss things. Additionally, some codebases are enormously complex, and it's virtually impossible for a single dev to understand it completely.