It is important to test on multiple platforms because you may have used a function (or a class of functions) from the Win32 API that is only supported on one platform. For example, the graphics device interface (GDI) in Windows NT supports a full 32-bit coordinate system, which Windows 95 does not support. If you are relying on 32-bit coordinate values when your application is running under Windows 95, you will not get the desired results.