"Corrupt endeavor" is a term of art that is derived from 18 USC 1503, Obstruction of justice. The phrase "corrupt endeavor" appears twice in the indictment, and the word "corruptly" appears once. All of these occurances are in the context of Count 1 of the indictment.
If what Libby did was merely pass on information obtained from Woodward and mistakenly attributed them to Russert, is that a corrupt endeavor?
In the context of this case, I would not think that sort of error would be material.
If the true source was the CIA and Libby wanted to mask that, a corrupt endeavor would be a desire to trick the investigators into thinking the information was derived from reporters. The motive being that unless he deflects the real source he might be guilty of a possible crime.
However, if the true source was not the CIA but actually was a reporter then can a corrupt endeavor occur? He no longer is motivated by hiding the truth of his source.