While its true that we won't hit this particular invariant anymore, there is still an issue with the output options parser in the new MR implementation. When building the agg pipeline from an MR, we bypass the "lite parsed" version of the DocumentSources, which has the unintentional consequence of bypassing several parse-time checks such as this one in $out. We should either move/duplicate the assertions to the normal DocumentSourceOut/Merge parsers or attempt to catch them in the translation layer before building the pipeline.