When Leonidas asks Queen Margo to marry him by writing "Marry Me" on her chest with a Sharpie pen, the camera changes positions a couple times during the scene. The words disappear each time the camera switches to the wider shot.
When Leonidas picks up the Subway sandwich, the top of the sandwich is in his right hand when he takes a bite, and in his left hand when he spits it out.
When the Spartans stop to "replenish" their electrolytes, Sonio is tossed a bottle of red Gatorade. When they pan back to him after showing a few of the other warriors, the Gatorade in Sonio's bottle is blue.
After Xerxes staples his nipple ornament back on, the scene changes, and when the scene comes back to Xerxes the staple is gone.
Leonidas is seen reading and writing early in the movie, but when Xerxes threatens to write Sparta out of the the history books, Leonidas replies: "Well, that's fine by me, Xerxes, because I can't read." He was just saying that to confuse Xerxes.
When Queen Margo flips over for her massage, white tape is visible on her breasts.
When Leonidas breaks the glass to get the penguin spear, he shatters it in the middle. When he reaches for the spear in the next scene, the glass has been broken from the end (to make grabbing the spear easier).
When they first sing "I Will Survive", Sonio's mouth can be seen singing alone, before we hear the song or see anyone else is singing.