Audio problem: When the Greeks are sacking Troy towards the end of the film, you can hear a soldier yell "Aquí" twice. Aquí is Spanish for here. Most of the movie was filmed in Mexico and used Mexicans as extras.
Other mistake: The umbrella used to shelter Paris and Helen during the parade in Troy is obviously modern, as it includes metal spokes like modern umbrellas.
Continuity mistake: When Achilles is burning Patroclus' body, the moonlight reflects slightly off his arms. While a body is burned, a person with a torch is always beside the one who places the coins, so the firelight would've shined off Achilles' arms as well, and yet it doesn't, indicating that the scene was not shot continuously.
Continuity mistake: After the Trojans were ordered to push forward after the Greek's first attack, blood can be seen suddenly appearing on a few of the Greeks shields.
Revealing mistake: When the soldiers of Greece are collecting the wood for the Trojan horse, you see bits coming of the wood, revealing the white Styrofoam used to make the wood.
Answer: He was warning the other soldiers that Achilles was coming.