Reaction 10, a substitution-based retrosynthesis, predicts a reduction of formaldehyded 2 with hydrogen to form the target alcohol. Not only is this a starting material with one carbon when our product has two, curiously, the second deuterium on the original precursor is completely absent in the predicted product. To probe this error, we queried a retrosynthesis of ethanol, which is similar to the target molecule albeit without the deuterium, shown in Figure . This resulted in a very similar set of predicted precursors, indicating that the model is simply trying to match these cases to a training example, and giving a chemically unreasonable result in the process. It is disconcerting that the predictive confidence of this ethanol synthesis is 1.0, further emphasizing that this is a reaction the model knows well and wants to apply in the case of reaction 10. Including a single deuterium atom results in the lower quality prediction result, highlighting a significant obstacle to sequence-based methods. Moreover, the predicted synthesis for reaction 10 completely fails when run through the forward prediction model.