Consumer protection is an important – but sensitive – area which allows for information to be conveyed l’toeles, for a constructive purpose.
If a storekeeper hands a customer too little change, or charges him for something he did not purchase, one must not assume that the storekeeper is dishonest or careless – we all make mistakes. If this happens often, one must bring the matter to the storekeeper’s attention and inform him that if the practice continues, one will have no choice but to inform his clientele. If the situation still does not change, one is required to warn people to count their change carefully and examine their receipts for any mistakes. It is forbidden to insinuate that the man is dishonest even if one has reason to suspect so, since it is sufficient for people to think that hs is careless (or has problems with his arithmetic) for them to take the necessary precautions.
It would also be forbidden to inform hot-headed individuals who, for example, might vandalize the store in the name of justice.
Sefer Chofetz Chaim, 13 Kislev, Page 194