When Phil admonishes Hercules, "Don't let your guard down because of a pair of big goo-goo eyes," he's clearly mouthing the word "blue" instead of "goo-goo." Meg's eyes were originally going to be blue, but later on they were changed to purple to match her dress.
When Hercules and Meg are talking after their day together, Meg is wearing a shawl. However, when she turns to talk to Pain and Panic disguised as birds in a nearby birdbath, her shawl is missing. It is there again when she is talking to Hercules.
When Hercules destroys the marketplace near the beginning of the movie, the pillars start falling outwards from the center of the circle towards the entrance of the marketplace. In the next part of the scene, the pillars are collapsing inwards towards the center of the circle and fall on each other before they crash on the pottery vendor.
During the scene with Megara and Hercules by the fountain, the armless statue at the end changes into a man and a woman holding a ribbon between scenes. Also, during that scene, Meg and Hercules change locations around the fountain and the statue rotates with them.
In the shot of the gods after Hades makes his joke about moussaka, Dionyssus is close to Aphrodite and Athena's owl is on her left hand. Then as Hades asks "Is this an audience or a mosaic?", Aphrodite is further from Dionyssus, and Athena's owl is on her right hand.
When the gods are walking down Mt. Olympus in chains, Hephaestus is in front of Aphrodite. But after Hercules cuts the chains with his sword and frees the gods, Hermes is in front of Aphrodite, and Hephaestus is nowhere to be seen.
Hercules meets Phil. Phil walks away & enters his tree. Hercules grabs the door & yanks it off it's hinges. You can see there's a hinge on the door & another was left in the doorway's frame. Phil is hanging onto the door handle as Herc holds it up. After Phil lets go of the door, the door has both hinges.
When baby Hercules is found, he's wearing a medal around his neck with a ribbon. The ribbon is tight around his neck, yet when the old man lifts the medal, the ribbon grows a couple of feet as he pulls it away from Herc's neck & turns the medal around to read it.
At the end when Hercules comes back to Mt. Olympus, there are two huge lines of gods greeting him on either sides of a long staircase. After Zeus and Hera have talked to him, about ten gods start mobbing him. When the shot cuts back to Meg the staircase is empty. Where did all the other gods go?
When Amphitryon finds the baby and looks at his medal, the word "Hercules" is written on one side. Later, older Hercules is talking to his parents, and his mother takes out the same medal to show to him. Look closely, and you will see that the word "Hercules" is gone.
In an overhead shot as the marketplace is tumbling down, several items that should be in the middle of the market are gone. This includes the wagon with the bundle of hay and the fountain that Hercules had sat next to a few minutes earlier.
Teenage Hercules is running with the hay-cart towards an archway. As he runs toward it, we see the arch is higher than the hay. In the next shot, we see the hay is much higher than the arch. Then in the following shot, the hay barely brushes under the arch.
Herc is fighting the Cyclops. He has two black eyes & a bruise on his left arm. Phil shows up & starts to wipe Herc's face. Suddenly, a bruise appears on Herc's right arm too. After Herc says, "Dreams are for Rookies", this bruise disappears.
Panic flies up in the air & lands with his horns stuck in Pain's butt. When he lands, he has a horn in each cheek. Then Pain stands up & turns sideways, showing Panic with both horns in only one of Pain's cheeks.
When Hercules tries to grab Meg's soul, his hands become very old as soon as they enter the water. But when Hercules is swimming to the bottom of the river, it is quite a while before his hands become old at all.
When Meg and Hercules are walking down the stairs in the garden, Meg has a sheer purple shawl, but when Pain and Panic (disguised as birds) tell her to stop fooling around, it disappears. Then, a few shots later, when she tells Hercules not to thank her just yet, it has reappeared.
While watching today, I realized right before "Go The Distance' when Hercules' mother is putting the wrap on him, originally it's green, but throughout the song, it's blue. Part of me thinks that it's supposed to be the shadows from the sunset, but looking again, it seems that it suddenly changes from green to blue. Especially since during the lightning storm in the song the color doesn't change much at all.
When Hercules opens the case, all that falls out are two pots, two bowling pins, a helmet, a sword, and two pairs of armor. A purple cape, two shields, many other pots, a sandal, a trophy, and a white ball with a blue stripe are among many of the additional items that appear out of thin air in the very next shot.
Before Zeus picks Hercules up, Herc's medal is untucked. Then when Zeus has him in his hand the medal is tucked in. When Zeus puts him back on the ground, the medal is gone, but Hercules has it again later in the movie.
When Hades says "Here I am kind of river guardian-less" you can see a chess piece of a centaur on the board near to a lot of other pieces. The when Meg flicks the centaur off the board, the other pieces are gone.
After Hercules slices off the hydra's head a lot of green goop comes out of its neck and forms a large puddle. A few seconds later, as the hydra suddenly sprouts three more heads, the puddle of goop is much smaller.
During Hades' meeting with the Fates, he creates a figurine of baby Hercules and puts it on the table. After the Fates reveal part of the future to Hades and fly away, in an overhead shot the figurine is gone.
During the song "Zero to Hero" a large golden statue of Hercules is unveiled in Thebes. Later in the film, when the cyclops is attacking the city, the statue is in the same location, but it is much smaller, and now it is made of stone.
After failing to kill Hercules, Panic says "Hades is gonna kill us when he finds out what happened". In this shot, Pain's tail is wrapped around his side twice. In the next shot, when Pain replies "You mean IF he finds out", his tail is wrapped around thrice.
In the scene where young Hercules is pulling the cart of hay, his's adopted father and the donkey are riding on the seat and both are holding onto the sides of the seat since they're going so fast. Once they stop, Hercules's adapted father mentions the donkey's hurt foot, and she's now got a sling on the left front leg which wasn't there during the wild ride.
When Hercules is running into the market with the wheelbarrow, as his dad cries, "Hercules, slow down," the donkey's leg is unbandaged and perfectly all right. This changes after Hercules arrives in the market.
Before becoming mortal, the infant Hercules's skin is much darker than his blond hair, which only turns red once he becomes mortal. Yet when he regains his godhood in the end, his appearance, hair and skin color are the same as they were when he was mortal.
Since this film is only an adaptation of the myths it is understandable for there to be quite a bit of artistic license taken with the stories. Any differences between this film and the myths belong in the trivia section of this site.
When baby Hercules goes missing, Zeus assembles all of the gods and sends them on a desperate search to find Hercules. The story says that they found him, but could do nothing since he was mortal. Although Hades is in the underworld and busy working, one would think he would listen for any news about Zeus/Hercules. In addition to the plea and the news of the search outcome, wouldn't the fact that Hercules was alive be common-knowledge amongst the gods? If Hades had heard either the outcome or spoken to another god in the 18 years Hercules was mortal, he shouldn't have been surprised to hear that Hercules was alive.
Mistake Hades' hair turns yellow, orange, and red when he's angry. Visible blue fire is the hottest flame, but also the cleanest i.e. no soot. The color changing could represent emotional soot blowing Hades composure.