During production on Alice, as was the practice on many other Disney animated films, actors performed some scenes in a studio for reference and inspiration for the animators. One such scene was the Mad Tea Party in which Ed Wynn (who plays The Hatter) and Kathryn Beaumont (who plays Alice) acted out their parts in costume. Wynn was a master at comedy and improvised a few bits. Walt Disney witnessed this and instructed the team to use some of the audio in the finished film. They attempted to have Wynn re-record his improvised lines in a proper recording booth as the audio in the studio was pretty bad (poor quality recording in general, and lots of extra noise as it was never intended for use in the picture) but these new takes were never as funny as the originals. So they tried their best to clean it up but you can still tell in a few of the Hatter's lines when they sound slightly poorer quality than everyone else's in the scene. So theoretically this leftover "great" should be from an animator or director in one of those improvised takes. But this line is almost a direct quote from book-- "begin at the beginning and go on till you come to the end. Then, stop." And it's set up by the March Hare further suggesting that it was already in the script. So either they also decided that this take of that line was better in this performance than in the booth, or Ed Wynn was just on the same comedic wavelength as Lewis Carroll. Either way, it's crazy that this "great" is still in there because it's not like it's occurring at the same time as anyone else's line making it harder to erase: it's all on its own. I don't have the latest blu ray release to check but it's definitely still there in the Disney+ version. This is all the info I could find on this anomaly, if anyone has any other information or had better luck than I researching it, please let me know! I've been fascinated since I discovered it with my daughter this weekend.