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.
After Juliet has run upstairs when Romeo and she decide to marry, the camera quickly pans up when she enters onto the balcony. When it does so, you can see the boom mic in the top of the screen quickly disappear.
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.
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.
Romeo falls from the balcony into the pool as the Nurse appears. He should have fallen hard on the pavement around the pool edge. He could not have reached the pool, even if he'd leaped out on purpose.
When Romeo buys the poison, he runs up some stairs to visit the old man and we see nothing against the wall. However, when he drives off in the car, we can see a sofa and some clutter leaning up against the wall.
In the outlands of Mantua Romeo calls out Juliet's name from his knees while facing the sun. He then stands and turns around. In the next shot, shadows indicate that he is walking towards the sun again.
After Romeo and Juliet spend the night together and she tells him that the light they see is not the sun, he rolls on top of her with the sheets billowing over his head. His shirt is first bunched up around his shoulders with his chest exposed, but is perfectly straight and covering his body in the next shot.
When Juliet's mother is looking for her daughter and running through the foyer, part of her nipple is exposed but in the next shot it is gone. Then, after speaking to Juliet in the dressing room she turns around with a cigarette and there are distinct black smudges on her chest. When she leaves the room, these smudges are gone.
At the Masquerade when Romeo and Juliet first meet, Juliet is dancing with Paris. Between them, in the background, is an extra with a very noticeable mustache, green mask, and mohawk. He seems to teleport behind Romeo during cuts.
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.