 First, an unpublished but much-discussed example due to Ned Hall. A hiker is walking along a mountain trail, when a boulder high above is dislodged and comes careering down the mountain slopes. The hiker notices the boulder and ducks at the appropriate time. The careering boulder causes the hiker to duck and this, in turn, causes his continued stride. (This second causal link involves double prevention: the duck prevents the collision between hiker and boulder which, had it occurred, would have prevented the hiker’s continued stride.) However, the careering boulder is the sort of thing that would normally prevent the hiker’s continued stride and so it seems counterintuitive to say that it causes the stride.