Competing Fault Histories in Lattice-Surgery Decoding

arXiv:2610.33591v1 Announce Type: new Abstract: Different physical faults can produce the same detector record while carrying opposite logical labels. We investigate the consequence of this ambiguity for a lattice surgery parity measurement. In a distance-three merge--split circuit, single-gate faults compete with reset/readout pairs on three complete syndromes. Full-syndrome probabilities give a simultaneous 95\% lower confidence bound of $16.64$ parts per million on the cost of retaining the selected single-fault decisions at one noise point. Changing those decisions is harmful at low noise and beneficial at higher noise. To test the physical explanation, we attenuate eight implicated channels and compare the change in override effect with a matched non-target suppression.