What will GetLastError() return after a failed InitOnceExecuteOnce?

The documentation for Init­Once­Execute­Once says

If the function fails, the return value is zero. To get extended error information, call Get­Last­Error.

On the other hand, the documentation for the Init­Once­Callback says

If the function returns FALSE, the block is not marked as initialized and the call to Init­Once­Execute­Once fails. To communicate additional error information, Gall Get­Last­Error. before returning FALSE.

The second paragraph implies that the Init­Once­Execute­Once function does not itself call Get­Last­Error, because if it did, then that would wipe out the value set by the callback. Is that really the case?

Yes, that's really the case. The remark in the first paragraph about calling Get­Last­Error presupposes that your callback uses Get­Last­Error to report why it wasn't able to initialize.

I agree, however, that the documentation is misleading here.

But if your callback wants to return status information more complex than a single "success/failed", you are probably better served by the synchronous two-phase initialization pattern, which makes it a lot easier to return more complex information. You can even use it to throw a C++ exception.