First linear mode connectivity (not named that until Frankle, Dziugaite, et al 2019) observation was by
@_vaishnavh and it was on test loss when swapping the training data. LMC in Frankle, Dziugaite, et al 2019 was defined on train, IIUC, but we definitely observed it on test (and think it was in the abstract). There's been a lot of cool work since then.