‘The Second Mistake’ (TSM) is to think that if an act is right or wrong because of its effects, the only relevant effects are the effects of this particular act. This is not (as some think) a truism, since ‘the effects of this particular act’ and ‘its effects’ need not co-refer. Derek Parfit's rejection of TSM is based mainly on intuitions concerning sets of acts that over-determine certain harms. In these cases, each act belongs to the relevant set in virtue of a causal relation (other than marginal contribution) to a specific harmful event. This feature may make an act wrong, in a fashion consequentialists could admit. That explication of TSM does not rely on the questionable assumption that the set of acts is what harms here. Independently of this, there are several other reasons to prefer it to the ‘mere participation’ approach.