In the final scene, when Krupa makes his comeback appearance with Tommy Dorsey's band, the set he's playing changes in the close-ups from the front left. The high hat cymbals are in a different position, and the tom tom to his left disappears completely.
In the scene between Gene and Ethel, right after Gene's mother argues with him in the speakeasy about the priesthood, Ethel's small handbag on the table is closer to her on the close-in shots than it is on the longer shots.
Krupa's father died when Gene was seven years old, so all the scenes between him and his grown son up to and including his death, are products of the script writer's imagination.
Krupa's drug bust, which occurred in San Francisco in 1942, has since been proven to have been a frame-up, but the film treats it as fact.
When we see Gene at his penthouse on Central Park, at the party one of the guests is Bix Beiderbecke. Bix died in 1931 and only ever played with Gene Krupa in Chicago. By the time Krupa made it in New York, Bix had already died.
In the spring and summer of 1930, Krupa made 7 records with Bix in NYC., most famously Barnacle Bill the Sailor in an All-Star band hand picked by Hoagy Carmichael which included Tommy Dorsey, Benny Goodman and Venuti & Lang.
The story begins in 1927, and continues on into the 1930s and 1940s, but all the women's clothing and hairstyles are strictly 1959.
The drums Krupa plays, beginning with the initial sequence in Chicago in 1927, are always of a later vintage and style than those used at the time being (incorrectly) represented.