One of the _good_ things about open sourcing parts of a larger stack of software is that it enforces protocols and API's in a much more thought-out manner.<p>I've also found that integration tests are a great thing here. If an internal application depends on certain behaviour, it's not likely that you'll have a test to begin with, but when the open source product conflicts with the internal, it's a perfect place to make integration tests to make both sides more stable.<p>I'm not a complete believer in all kinds of tests at all times, but I've found that the more extensive your tests are around things like this, the less breaks in the long run.