The actor playing Hercules whispers the next line to his opponent's actor at 0:53, as if they were on stage, and it isn't even edited out. Diegetically it would mean that it was Hercules's idea to fight to the death and pretends it wasn't.
"There's nothing around here except the warrior's training ground" he says after overtaking his new friend in the middle of nowhere that just happens to be equipped with a pool complete with decking.
One warrior has a tattoo in the shape of Texas. The Rio Grande would have been unknown to Ancient Greek mapmakers, and only a very good oracle could have foreseen where its boundaries with New Mexico, Oklahoma, etc. would be drawn centuries later. To remove all doubt, it's labeled with the letters Tau Chi, which look a lot like the postal abbreviation for Texas -- invented in 1963.
A string of electric lights is clearly visible in the background of the hot tub scene at 0:37.
A warrior has a tattoo running down his side consisting of three words -- in English, which would have been an obscure foreign language in Ancient Greece.