If you want to get technical, it the ob hit the back of the pocket, it was "in the pocket" at one time. The fact that it came out again doesn't matter.
IMO, no foul.
Steve
According to the written rules, which are available on the WPA website, you have a misconception. Here is a brief quote from those rules:so you slam a ball into the pocket, it hits the back of the pocket , jumps out on the table, and it never hits a rail, is it a foul....because a ball mustbe pocketed or hit a RAIL..... the back of the pocket isnt a rail?