Not sure that this actually qualifies as a bug, but it was unexpected behavior. For unit testing I was creating an empty Workbook document and writing it to a file. It turns out, that the file is corrupt unless it has a Worksheet added to it. The XlsxFormatProvider does not issue any error and happily writes a 3k file, but Excel can't read it. Excel throws a dialog "We found a problem with some content in the 'test.xlsx'. Do you want us...". Selecting Yes eventually comes back with a "The workbook cannot be opened or repaired by Microsoft Excel because it is corrupt." message.
Simply adding a worksheet:
will allow it to create a file that Excel will handle. Again, not sure you could call this a bug, but it wasn't what I expected it to do. It might be better if the XlsxFormatProvider threw an exception for trying to write out an empty file.
At any rate, figured I throw this out there in case anyone else runs into this behavior.