When the Ritchie Valens character charges Buddy his shoes fall off very obviously, but they are suddenly back on his feet in the very next shot.
When buddy fights the goons while the kid is placed in the trunk of the car, the natural light changes between shots.
The hole that Death's archers put in Buddy's guitar shows up earlier in the film as Buddy and the kid are rolling off of the motorcycle.
When Buddy is walking toward the archers toward the end of the film we see the kid running behind him trying to bring his sword to him. However, you can clearly see Buddy's sword still attached to his guitar.
The guitar being used by Buddy throughout the film is referred to as a "1957 Six String Hollowbody". The guitar is actually a 1965-1966 Martin GT Series Hollowbody guitar.
Some rifles that Russians use, are obvious rubber props.