Yesterday I heard (on TV) an American native speaker say: "Best intentions doesn't make our dog better." One might expect that she should have said: "Best intentions don't make our dog better", but what she said wasn't a mistake.
Generally, I am sceptical of linguistic explanations that account for a phenomenon by alluding to something supposedly implicit, not present in the context of the phenomenon in question. However, in this case, I'm inclined to claim that best intentions implicitly is understood as the concept/idea of best intentions, and that the verb (to do) of the sentence correlates with the concept/idea in number (sing.): "The concept/idea of best intentions doesn't make our dog better."
This phenomenon is similar to one present in Norwegian, where it is perfectly acceptable to say, for example: "Pannekaker er godt" ("Pancakes[masc.plur] is/are good[neutr.sing]" (we don't conjugate the verb according to the subject in Norwegian)). One might expect us to say: "Pannekaker er gode" ("Pancakes[masc.plur.] are good[plur]"), but this construction is actually uncommon. Again, I think that the explanation is that we treat pannekaker as a single concept, as opposed the specific instances of pancakes.