Skylar Neil's age is inconsistent throughout the film. In real life, she was born in 1991 and she died in 1995 at the age of four. The film implies that she was born sometime in 1985, yet she appears to be three or four years old in the 1987 scene where Vince reacts to the news reports of Nikki's overdose. Furthermore, she appears to be four or five years old in her 1995 death scene.
Tommy Lee did not meet Heather Locklear at a party in Vince Neil's home in 1984. In real life, they met backstage after an REO Speedwagon concert after Lee's accountant introduced him to her.
In real life, Vince Neil's return to Mötley Crüe did not immediately lead to a happy ending. The band was pressured by Elektra Records to fire John Corabi and bring back Vince following the commercial failure of their self-titled 1994 album. Following their 1997 reunion, the animosity between Vince and Tommy Lee persisted until Tommy quit the band in 1999, forcing the band to replace him with Randy Castillo for their 2000 album New Tattoo. Tommy subsequently rejoined the band in 2004.
Tommy Lee is playing Zildjian cymbals throughout the movie, but he was a major endorser of Paiste cymbals from 1981 to 2004, so most certainly wouldn't have played ZIldjian in public if at all.
When Nikki Sixx changes his name and gets a new driver's license at the BMV, he sets fire to his old license that has a magnetic strip on the back. Those did not exist in 1978.
In the opening scenes you see Nikki (then Frankie) dancing around his room with a six string guitar. According to the book, The Dirt, Nikki had never owned a guitar before auditioning for his first band in high school. The day of his audition, he took an empty guitar case to a music store and stole a guitar. When he got to the band audition to try out as their bassist, they noticed he had a regular six string guitar. When they questioned Nikki about it, he told them, "I know. I play bass on guitar."
While the movie suggests Doc McGee was fired because he brought Nikki's mom to see him, Motley Crue fired McGee in 1989 after the Moscow Music Peace Festival. The band thought McGee was favoring another band he was managing, Bon Jovi, by giving them top billing and letting them use fireworks in their show.
In the scene where Nikki and Tommy audition band members in 1981, Tommy wears a Hulkamania t-shirt. Hulkamania was not coined by the World Wrestling Federation until 1984.
Fast Times at Ridgemont High poster visible outside the Whisky in a 1981 scene; the film wasn't released until 1982.
Young Frank/Nikki has KISS albums and posters in his bedroom in 1973. KISS' debut album, seen in his room, wasn't released until early 1974.
When Heather Locklear dumps Tommy Lee in 1993, he is wearing a modern pair of AKG headphones.
Nikki Sixx's old Washington State driver license is plastic with a picture on the right side. In the late 70s and early 80s licenses were still made out of paper and pictures are on the left side.