Revealing mistake: When the doctor intercoms the colonel because he cannot get into the medical room for the pregnant woman having nightmares, the camera shows the intercom, which is simply the back end of a desktop computer with a couple of knobs stuck on it. (00:24:50)
Revealing mistake: Hercules walks out at night and suddenly encounters Megara in the forest. Directly afterwards you see them walking off in the distance. The wall behind them has some torches on it, but apparently these were added in post production: One of them shines right through their bodies as they walk past. (00:47:00)
Plot hole: Throughout the movie, the flies are vulnerable to light (direct sunlight and even lightbulbs) to the point of incinerating in a split second. Except... they are not; in several sequences they fly to and from bodies and without even taking a direct path (look at all the action happening in front of the house, by the cars, in broad daylight).
Continuity mistake: During the chase in chariots (2 Romans against Astérix), when the palace is attacked, in one shot, we can see there is only one Roman soldier on his chariot instead two. (01:21:55)