At the beginning of the episode, Grandpa was telling how he met Arthur, and the camera will show the duel, and you can see that the Millennium Shield was in attack mode, not defense mode.
When Rebecca used Stop Defense on Yugi's face-down defense card, a second before the card flipped, Tristan exclaims, "She's forced the Dark Magician into attack mode!", How did he know what card it was?
At the end of the episode, when the 'Ties of Friendship' falls onto the Blue Eyes, you'll notice that the Blue eyes no longer has tape on it, as though it is brand new.
Because the Dark Magician appeared before he said that.