Parshah: Huqqat, "Mixed Messages?"

It seems that many think that Moshe Rabbeinu's error at mei meriva was to hit the rock instead of speaking to it. 

That seems pretty pashut! However, when Hashem tells Moshe "aseih lecha saraph,"  Moshe Rabbeinu actually: "Vaya'as Moshe Nechash Nechoshes.."

Now Hashem had said "Saraph" and Moshe instead made "Nechash nechoshes." Is this not, too, a deviation - albeit minor - from Hashem's statement? Does anyone comment on this apparent contradiction?

Shalom, RRW