(1) Remove pairs with non-numeric elements from `neg` and `pos`, (2) `Prepend` (`Append`) `neg`  (`pos`) with a point so that both series have the same horizontal and vertical ranges:


    Show[{QuantilePlot[X, NormalDistribution[Mean[X], StandardDeviation[X]], 
       ImageSize -> Automatic -> 200, LabelStyle -> 12, PlotStyle -> Red],
       ListLinePlot[{Prepend[{-3, -3}]@Cases[neg, {__Real}],
          Append[{3, 3}]@Cases[pos, {__Real}]},
       Filling -> {{1 -> {2}}}, 
       FillingStyle -> Directive[{Gray, Opacity[0.1]}]]}, 
     PlotRange -> {{-2, 2}, {-2, 2}}, ImageSize -> Large]


[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/IVkjN.png