I found this one out the hard way today. I was experimenting with the KMDF loader driver (wdfldr.sys). I added the following #define to my sources file so that I could share code between wdfldr and another component and control some functionality based on who was including the file
sources:
    TARGETNAME=wdfldr
    TARGETTYPE=EXPORT_DRIVER

    C_DEFINES=$(C_DEFINES) -DWDFLDR=1

    [...]
I then compiled my test driver (wdfrawbusenumtest.sys), but it failed to load! I was getting a code 39 (which meant Windows could not load my driver). I previously wrote on how to debug a missing export at runtime, and I tried to use that technique. It told me that "KeGetCurrentIrql" was unresolved, but that is not possible since hal.dll has exported this function since NT 3.1. It finally occurred to me look at the imports for my test driver to see if there was anything weird. Here is what I saw:
    link /dump /imports WdfRawBusEnumTest.sys

    [...]
        HAL.dll
                     18014 Import Address Table
                     1C290 Import Name Table
                         0 time date stamp
                         0 Index of first forwarder reference

                       4C KeGetCurrentIrql

        1.SYS
                     18000 Import Address Table
                     1C27C Import Name Table
                         0 time date stamp
                         0 Index of first forwarder reference

                        7 WdfVersionBind
                        9 WdfVersionUnbind
Huh? 1.sys? Where did that come from? I went back and checked the src file (which when compiled becomes the def file) and there were no changes there. This is what the def file looks like:
wdfldr.src:
    NAME WDFLDR.SYS

    EXPORTS
        WdfVersionBind
        WdfVersionUnbind
        [...]
Since I didn't change the def file, what was making the compiler think that the exports should resolve to 1.sys? I finally realized that my C_DEFINE of -DWDFDLR=1 was the culprit. When wdfldr.src was being built into wdfldr.def, my define changed NAME WDFLDR.SYS into NAME 1.SYS. Little did I know that the C preprocessor was used when compiling the src file. To fix the problem, all I needed to do was change the define to something else, C_DEFINES=$(C_DEFINES) -D_WDFLDR_=1, and then update my shared headers to use the new define.