I think we're talking about the pause in the dialogue, as in: "Scar ... brother, help me." The "mistake" is apparently that we hear the word "brother" twice, even though I don't recall that since I only watched the movie once before 2004; maybe that was in the original English theatrical release or VHS? As for why they might have opted not to do a pickup session -- maybe they decided they liked the resultant hesitation? As if Mufasa doesn't really want to refer to Scar as his brother but, after a pause, thinks that he ought to?