When the camera zooms in on the newspaper article detailing the story about the film crew coming to the small town of Embry, Indiana to film a movie, the website www.embrystar.com is clearly visible under the name of the newspaper - the article was supposedly written and published in 1982, eight years before the first website existed.
When Harry discovers a small pistol in the glove compartment of Perry's Mercedes, Perry refers to it as a Derringer, and even specifies (by way of a joke) that it only carries two shots, as Derringers invariably do. However, it is clearly visible as a North American Arms mini-revolver.
When Perry and Harry are on the rooftop having an argument,
Perry puts his hand across his mouth to shut Harry up with his right hand. The next shot centering on Harry has Perry's left hand on Harry's mouth, and the shot after that returns to having Perry's right hand on Harry's mouth.
In the voiceover, we are told Harmony came to LA from Indiana, but in the footage of the bus going down the highway, it turns onto GA-16, outside of Marietta, GA. There's no reason she would take a bus that far SE to head to Los Angeles.
When Harry answers Perry's mobile phone towards the end, the screen of the phone is clearly visible. Instead of showing "Harmony Calling" which would be correct, the phone display simply shows the phone book entry for Harmony instead.
The book that Harmony takes off the shelf at the party is entitled You Wouldn't Want to Live There by Joe Chester, a fictional detective novelist. When she opens the book to read it, the text at the top of the pages clearly states that the novel is Maureen Child's The Man Beneath the Uniform, a 2004 Silhouette romance novel.
The goof items below may give away important plot points.
When Pink Hair Girl is shot, she hits the floor with her face turned toward Harry (and the camera). In the reverse shot immediately following, she is facing the ceiling, then turns her head toward Harry. (This same sequence repeats in the recap flashback.)
When Harmony appears at Harry's hotel room and begins talking about how it wasn't her that died, but in fact her sister, she swipes an empty champagne glass from Harry's hand, trying to drink from it. However, the glass actually had some liquid left in it only half a second before hand after Harry had finished taking a rather large swig.
After Harry and Perry begin moving the the corpse from Harry's hotel room, the length of the corpse becomes significantly shorter once they get into the hallway, and remains shorter as they heft it into the trunk of the car. This is given away from the actors tucking the blanket too tightly around whatever prop they were using.