In the replication machinery, during the polymerase dNTP sampling, the sample is rejected if G is mismatched against T. We investigate a pathway that connects the wobble mismatch to a Watson-Crick-like pairing (shown as pathway 1 in Fig. ), leading to base misincorporation through a phosphodiester bond formation. In this scheme, proton transfer must occur for the bases to slide to a Watson-Crick-like pairing, either classically, via an "over the barrier" mechanism, or via quantum tunnelling. To avoid replication errors the polymerase should reject such mismatches; otherwise, the wrong base pairing can undergo further proton transfer, connecting two Watson-Crick-like tautomeric forms (shown as pathway 2). Additional pathways are explored in supplementary note 3.