Edit: Possible spoilers. If you're a baby about that sort of thing and smallest details ruins you, don't read this. This is more for Bethesda to read anyway. Also if there's a proper bug report channel somewhere, Leave a reply in this thread and i'll submit it through that chanel. But I couldn't find one.

In the Quest "the big dig" , specifically the part where Bobbi asks you to break Mel out of Diamond City security's jail, there's a bug where you don;t actually have to do anything to free Mel. See I wasn't sure I wanted to Help Bobbi, but I wanted to check it out anyway. So I went over to the jail, looked around (without picking any locks or talking to anyone), then I left. When I left I got a quest notification and the next phase of the quest begun. Moments later, Mel, who was just behind bars, walks out of the Security Building as if I had freed him.

Bug: Mel get's freed from jail without having picked any locks, or bribed any guards.

Repro: I haven't tried to reproduce this bug. But basically start up "The Big Dig", get to the part where Bobbi Asks you to meet her in Diamond City and talk to her there. Then head to jail, do nothing, and leave.

All in all a pretty benign bug, but Bethesda should care nonetheless and fix this.

Spoiler alert:

you make an interesting point, However this is why it's not the case: After Mel was "freed" I spoke to him and he talked about how (paraphrasing) "Bobbi is so impatient, my sentence wasn't that long anyway, she could have waited". The dialogue doesn't make sense if he had served his time and was released. He spoke as if I had freed him.

It happened to me as well, entered security then exit as I had second thoughts...and before you know it, he freed himself and thanked me for pulling it off.