But because of parallel integration of source code modules there is a combination of source code which has not been tested together before. Numerous integration problems arise without detection. Further problems arise when there is no clear cut latest version. This applies not only to the source code but the unit test suite which must verify the source code correctness. If you can not lay your hands on a complete, correct, and consistent test suite you will be chasing bugs that do not exist and passing up bugs that do.
Some projects try to have developers own specific classes.
The class owners then ensure that code for each class is integrated and released properly. This reduces the problem but interclass dependencies can still be wrong. It does not solve the whole problem. Yet another way is to appoint an integrator or integration team. Integrating code from multiple developers is more than a single person can handle.