When Mercutio and Benvolio are at the beach, the shirt that Mercutio is wearing is made of thin silk-like fabric and is see-through. After Mercutio has been killed, his shirt is made of a thicker linen and is not see-through.
The vial of poison falls from Romeo's hand when Juliet touches his cheek, but later she takes it out of his hand.
When Romeo learns of Juliet's death in the desert, his shirt is closed. Then when Balthasar goes to comfort him and Romeo pushes him away, his shirt is pulled open. Then later, after Romeo wails whilst crouching down, he stands up and his shirt is again closed.
When Romeo is holding Mercutio in his arms as Mercutio puts a curse on both houses, Romeo has blood on his collar and left sleeve. When Romeo stands up the blood is gone.
The poem book disappears from Romeo's hand when he stands up.
When on the beach preparing for a duel, Abra ejects all the
bullets from Tybalt's magazine except one. Romeo uses that same gun, in a new location, to kill Tybalt, shooting him 6-7 times. However, Tybalt carries two guns. The one Romeo uses is the second gun, which at this point was not unloaded.
The movie is set in fictitious Verona Beach, Florida, not the landlocked Verona, Italy, which has no beaches.
When Mercutio is dying on the beach, the digital clouds are blowing sideways, but on the ground, you can see the shadows of real clods coming toward the camera.
When we first see Juliet's house, the fountain footage is running in reverse.
(Around 15/16 minutes) when Juliet emerges from the bath, the top of her swimsuit can be seen for a few frames.
During Desiree's song at the Capulet party, the violinists are never shown playing. At the beginning and end of the song, the violins are on their shoulders, but during the part of the song with strings, no violins are visible at all.
When Mercutio fires his gun at the beach as the Nurse is leading Romeo away to talk to him, there is a gunshot, but no flash or blowback.
When Mercutio is shooting fish at the beach, his gun is heard firing, but there is no flash.
While driving away from the gas station, Gregory is shot in the shoulder. No mention or visual hint of this is given again throughout the remainder of the film.
At the gas station showdown, a boom mic is reflected in a car window as the camera moves past it.
When Romeo kills Tybalt, the shadow of the boom mic is clearly cast on him.
After Juliet runs upstairs when she and Romeo decide to marry, the camera quickly pans up when she enters onto the balcony. When it does so, the boom mic can be seen at the top of the screen and quickly disappears.
Gregory and Sampson were Capulets in the novel, not Montagues as portrayed in the film.