In Russian there no differences between “can” and “may”. I know the rule about these modal verbs, but sometimes I make mistake automatically. I just can’t deceive my unconscious.
By the way, what way is right, if I want “rebel” (Как ты смеешь/можешь говорить это?!):
1) How can you say that?!
2) How may you say that?!
Problem is in that, the Russian phrase has two meanings. I may say «смеешь» (“to dare”), and in this case, I think “may” is right choice. However, I may put another aspect in my words, and then this phrase would mean «[Да,] как твой язык поворачивается говорить это?!» (“How does your tongue turn [here means “moves”] to say that?!” — the common Russian indignation). So, I think the “can” is a more close verb in this case, but is it right to say "How can..." in English?