Revealing mistake: When Marius is talking to Lestat on the beach, the victim that Lestat drops is a dummy. (00:24:05)
Audio problem: In the scene during the flashback of when Lestat was first created during the shot with the girl on the beach, Lestat begins to play with her. When there is a close up of the woman's face, you can see her bow strokes. However, the strokes don't match up with the audio on the song you hear. (00:26:30)
Revealing mistake: Just after the flying scene when Lestat and Jesse land on the rail of the building, when Jesse falls you can tell for a few seconds that she is standing just below the view of the camera because her hand is just sitting there before Lestat grabs it. (00:58:00)
Continuity mistake: When Lestat goes to attack the woman in the park he already has blood on his mouth. (This is because originally Lestat attacked her twice, but this scene was later cut down.) (01:00:55)
Continuity mistake: When Lestat bites Jessie's neck her dress' stripes are down to her elbows, when he lets her down, they're up to her shoulders. (01:22:40)
Character mistake: In the scene in David's office, David tells Jessie that Marius is as "close as we have come to the original vampire" then gives her Lestat's diary which states that the original vampires are Akasha and Enkil.
Continuity mistake: In the scene where the Akasha confronts the elders, when Akasha enters with Lestat her hair is down in the back. When Akasha is disintegrating all of her hair is braided on the top of her head. This is due to the death of the actress/singer Aaliyah. They had to use what they could from other takes during the film process. Also a double with some digital re-enhancement helped with finishing the film.
Answer: There are already way too many characters in this movie. Adding the character of Magnus who would only be mentioned in passing was probably just considered too confusing.
Myridon