This is fairly obvious. When Dr. Hibbert and the Simpson family are talking about Hugo, Bart pulls up his shirt to look at his 'new found' scar on his right side. Yes, it is supposed to be the suprise ending that Bart is the evil twin. However if you look at the very next frame which is behind Bart, you see that he actually pulled up his shirt on the left side, and that the scar could not be seen with the shirt hiding it on the right side. Perhaps this was an older scene created then they changed it, or simple carelessness.