The slip-up occurs in the scene where the Trojans become aware of the approaching Greek fleet. The alarm is sounded and you see people running around the marketplace of the city. It's chapter 11 of the DVD, approx. 34 minutes into the movie.

On the left, you can clearly see a man and a woman leading away two llama's! Llama's are indigenous to South-America and could never have been present in Troy at the time.