Complex sentence: Since many C/C++ source files for Windows programs include stdafx.h, making small changes in it means the entire project needs to be recompiled and small errors in it cause large errors in the rest of the project.
Split into simpler sentences: Since many C/C++ source files for Windows programs include stdafx.h, making small changes in it means the entire project needs to be recompiled. Consequently, small errors in stdafx.h can cause large errors in the rest of the project.