When Jimmy's dad tells him that Elvis is God, Jimmy said he never pictured God with a fat gut singing "My Way" at Caesar's Palace. Elvis actually performed at The Frontier and Las Vegas Hilton, but never Caesar's Palace.
When some members of the group are talking about some famous Rock Stars have died, "Outspan" mentions Cass Elliot cause of death as a heart attack. The others correct him by saying it was by choking on her own vomit but it was indeed due to a heart attack.
As Andrew Strong is about to go into the main Chorus towards the end of "Try A Little Tenderness", when the drums kick in, the piano player goes for a strike down the keys, but in the next shot he's playing it normally.
When "The Commitments" play in their last gig, there is a big brass accompaniment. They are supposed just two. It's best heard during "In the Midnight Hour": there's a tenor saxophone from nowhere playing.