In the Capture the Flag scene, a warrior of the red team shears off the blue horsehair plume on Percy's helmet with his sword. However, in all of the following scenes the plume is complete again.
After Percy enters the Lotus Casino in Vegas, the designs on his blue shirt are suddenly gone.
At Auntie Em's Emporium her lip color changes between scenes from red to purple.
When Percy is in the Demigod infirmary after the fight with the Minotaur, there is a shield seen leaning against the table next to him as he sleeps. He is then woken up by Grover, the camera angle return to Percy sitting up, and the shield is gone.
Pigeons outside the window of Percy's house disappear and reappear between shots.
The statue of Athena in the Parthenon in Nashville does not resemble the actual statue. The real statue is gold plated, has a different headpiece and is holding Nike not a spear.
The heroes escape the Lotus casino by jumping into a Maserati that is offered as a prize and driving off. Due to fire danger the car on display would have had no gas in the tank so it could not have started and run.
Like in many modern-day depictions, the Underworld is depicted as a hellish place full of fire. However, in Greek mythology, all the dead went to the Underworld, their treatment differing on their behavior on earth. So while there was a place where the sinners were tortured, the Tartaros, the Underworld also had the Elysium, which was more or less a paradise for the good souls, the Mourning Fields for people who died while yearning for unrequited love and the Asphodel Meadows for everyone else. Also, the Tartaros was not simply a place of fire, since this is a depiction of hell which only became popular over the last few centuries.
In the Palace of the Gods on Mount Olympus in the throne room, there are 14 thrones shown, however, there are only 12 Olympian gods and goddesses.
Persephone says that Hades was cruel and abusive. In Greek mythology, Hades loved Persephone and treated her like a queen. Also, Persephone came to love Hades and the two ruled the Underworld together as equals.
In the opening sequence, Poseidon tells Zeus it has "been many years." In the Percy Jackson series, the Olympians held a council each year on the summer and winter solstices, meaning they would have seen each other in the last six months, but the films make it clear that they do not follow the same continuity as the books.
When Percy, Annabeth and Grover come to Luke to ask for help he is playing Call of Duty: Modern Warfare 2 (2009) with a Playstation 3 controller while on screen Xbox 360 buttons are seen.
Before Medusa takes off her turban, a small piece of Uma Thurman's blonde hair visible peeking out of the right side of the turban.
When Chiron in centaur form walks, he looks strange - but it's not because of bad CGI. It's because a horse, like all four-legged animals, walks with a four-beat gait; whereas whoever animated Chiron clearly didn't know this and tried to force the "walk" into a two-beat, unnatural gait.
During the fight with Medusa, she knocks over a row of statues which fall on Percy, trapping him. As he tries to free himself it is clear that he was not stuck; none of the statues was even touching him.
While the pearl is spinning on the roulette, and the pearl is behind the handle that sticks up, you still see the green pearl reflection as if it never moved.
During the Parthenon sequence, after Percy touches down after getting the pearl, the wings on his shoes are absent. This is more notable when we are shown them fighting around the pillars from long shots, shots from behind the men when they are talking together and when he is rolling/running across the ground.
When the group realizes they are short one pearl to get everyone out of the Underworld, it's never explained why Persephone cannot simply create another one for them.
The substitute teacher, Mrs. Dodds, writes, "I understand a fury in your words But not your words" on the blackboard, when the actual line from Othello 4.2 is, "I understand a fury in your words. But not the words".
When Percy discovers how long they were inside the Lotus Casino, he says that their deadline is at midnight the next day, after which Zeus and Poseidon will go to war unless the bolt is returned to Olympus. However, the purpose of the heroes' quest was to reach the Underworld to rescue Percy's mother - none of them know anything as to the bolt's location at this point, nor was it their intention to find and return it to Zeus in the first place, meaning the summer solstice deadline shouldn't be an immediate concern to them.
When Persephone summons the hell-hounds, she says "ela tho", which is supposed to mean "come here." However, this command is grammatically wrong. The correct Greek term is "ela edo."