When Boris came to collect the brief case in Snatch, he sees the diamond outside the brief case, and a minute after he requested it they answer that the stone is inside the brief case and "the only man who knew the combination" is dead.
So how was the diamond outside the brief case a minute ago?
I found related goof but still no explanantion
When Boris the Blade visits Vinny and Sol in the pawn shop, Vinny holds up the diamond. In the next shot, Boris the Blade asks Sol where the diamond is, and Sol says it is in the locked briefcase. Vinny had time to put it there (and can be seen doing so) and lock the briefcase, though it was admittedly not a very bright thing to do; then again, Vinny isn't very bright.