In 8-ball after ball groups have been designated.(no longer open table), A player mistakenly calls a shot and pockets his apponent's ball but "Foul" was not called. The offending player then called a second shot and pocketed another ball from his apponent's group of balls. This time Foul was called. My question is what rule prevails?
It may depend on what rules you are playing with, but typically if no one notices a previous foul before the next shot is made, it is no longer relevant. Here is an example of this rule:
11.1 Wrong Balls Pocketed
When it is discovered that the shooting player has been shooting the opponent’s designated ball(s) as if it were their own, the shooter shall relinquish the table with a ball in hand foul to the opponent (see 7.3, Bad Hit).