Revealing mistake: Wait until the end of the bonus footage of the Spice Girls' live performance of "Mama". At the very end of the song, the music and voices fade out. Sporty appears to throw her body and soul into one of the lyrics, even though the lyric is barely audible because of the fadeout. It actually looks pretty funny.
Other mistake: When the girls are chatting on a blanket outside the "haunted" castle, you can see Geri discreetly mouthing other people's lines, particularly Emma's.
Revealing mistake: When the Spice Girls are in dance camp, they go through an obstacle course. On the second obstacle (the tunnel), you can see that Scary Spice runs to the side of the tunnels instead of actually going through. When the shot changes to when they are exiting the tunnel, it shows Scary Spice crawling out.