The rules of etiquette are there to make others feel comfortable in social and business settings. If you are in the presence of someone who breaks an etiquette rule, do not embarrass them by pointing out their mistake. Instead, use your etiquette knowledge to come up with an appropriate plan ‘B’.
For example, if the person seated next to you mistakenly uses your bread plate, leaving you without one, say nothing. Use the rim of your dinner plate as a substitution.