When Xena tells her horse Argo to run off alone, the numbers '24' and '8' are clearly visible in black on the horse's shoulder, in modern script rather than the mock-Greek script used throughout this series. The numbers are notably absent in Argo's other scenes.
The Battle of Marathon happened in 490 BC, while the Spartan stand at Thermopylae happened in 480 BC, yet are implied to happen concurrently here. Admittedly, this is one of the smaller quibbles for a series where one woman has managed to meet Helen of Troy, David and Goliath, Julius Caesar, and Boudicca.
Xena speaks of "the Spartans helping the Greeks". They were among the oldest Greeks.
Gabrielle and Xena talk about what they did "in China." But in The Debt (1997)/The Debt II (1997) where that happened, the kingdom was called Chin, not China.