I've never personally experienced it because I've never bothered getting into a lounge I don't have access to. I can pretty well guarantee that there are agents that grant exceptions. I saw a GA let an LH*S in Economy board with Zone 1, when he was actually supposed to board with Zone 5. Likewise, there will be (a very limited number of) lounge agents that are lax/don't care about the rules that will let an I passenger into the SS. It's not fair to those who paid for a ticket with access, but the only way to stop it is to put a machine-operated gate in front of the lounge (which looks terrible).