Another's words or deed may be the cause of another's sin in two ways, directly and accidentally. Directly, when a man either intends, by his evil word or deed, to lead another man into sin, or, if he does not so intend, when his deed is of such a nature as to lead another into sin: for instance, when a man publicly commits a sin or does something that has an appearance of sin. On this case he that does such an act does, properly speaking, afford an occasion of another's spiritual downfall, wherefore his act is called "active scandal." One man's word or deed is the accidental cause of another's sin, when he neither intends to lead him into sin, nor does what is of a nature to lead him into sin, and yet this other one, through being ill-disposed, is led into sin, for instance, into envy of another's good, and then he who does this righteous act, does not, so far as he is concerned, afford an occasion of the other's downfall, but it is this other one who takes the occasion according to Romans 7:8: "Sin taking occasion by the commandment wrought in me all manner of concupiscence." Wherefore this is "passive," without "active scandal," since he that acts rightly does not, for his own part, afford the occasion of the other's downfall.
(Thomas Aquinas, Summa Theologica, 2a 2ae, q. 43, 1, ad 4)
And there's a line from one of Flannery O'Connor's letters that I'll post when I find it again.