Alcohol question - could either of us have gotten in trouble?

I live in Montana. I am not worried about anything happening right now because too much time has passed, but I am curious if something could have happened at the time.

Back when I was 18 (I’m 29 now), I went to a friend’s house to visit with him and his parents before we went to our graduation party. While we were visiting, the parents let me have a beer. I paced myself and after a while I left.

In this scenario, I was not pulled over, didn’t have any issue with police, nobody got hurt, no damage was caused to property, everybody at the gathering made it to the graduation party, and the party itself went off without a hitch.

Could either myself or the parents have gotten in trouble in this situation? If so, how? And how long after the fact would trouble have been possible? Would the cops be able to do anything solely on somebody else’s word? Could someone report the particular event after the fact?