We need additional logic on the QO side to ensure that SBE does not need to worry about merging conflicting paths for arbitrary QSNs. An example query would be: ($lu = $lookup + $unwind on as)

{$lu... as: "a"}

{$lu... as: "a.b"}

If we were to do the second join first, we obviously need to be careful not to overwrite field "a.b" when we do the second join (especially if field "b" contains something!)

For now, the easy solution is to bail in such cases.