Interesting. I always followed the law, so I suppose I wouldn’t have noticed a decoy who was handled properly.
I did, once, receive a call from a woman who wanted me to know that she had had an under-age young relative living with her who had bought wine from me. We he moved out she found a receipt. I wasn’t sure what that call could been about (under the surface) but I told her we always carded when anyone looked under 30 (the CA law), so was it possible he had a good fake ID? “Oh”, she said, “I suppose he probably did”. Closest I ever knowingly got to a legal issue, except for the few times I had to stop serving people who had clearly reached their limits.