When Eugene is walking up to the stage for the guitar duel at the end, we hear the sound of hard-soled shoes on the wooden floor. But in wide shots we see that he is wearing the same Nike tennis shoes that he's had on for the whole movie.
When Eugene and Willie are drinking whiskey after France's departure, Eugene starts playing on his Telecaster. You can clearly hear a "tremolo effect" which is impossible because the guitar is plugged straight to the amp and "Pignose 7-100" amplifiers don't have any effects.
When Willie and Eugene are buying the Telecaster in the pawn shop, they saw off a piece of metal tubing to make Eugene a "bottleneck" to play guitar with. But later in close-up, he is using an actual glass bottleneck.
Early in the film, Eugene puts a Robert Johnson tape into his tape player. The first track is listed as "Crossroads Blues." It should be "Cross Road"; the song title didn't become "Crossroads" until Cream covered the song in 1968.
At just about an hour into the film, in Crupps bar, the bartender pulls a shotgun to break up the fight. He racks the pump, because that is a sound movie audiences associate with a shotgun. The only problem is that the bartender is holding a double barrel over-under shotgun that opens up at the back, it is not a pump. And it will not make that sound.