So I took Cait with me to storm the Castle; naturally, Preston was already waiting there with his troops. Then a strange thing happened.

The quest refused to recognize that I'd completed it. It insisted I had to finish cleaning out the enemies in the area... enemies who were all cooked meat by now. I saved, restarted from old saves, and finally just wandered around for a good 20 minutes trying to do something that would trigger the next step before giving up and taking off in search of something else to do.

Then I realized that Preston was still following me, outside the Castle. I, Cait, and he have been travelling for a day now in real life.

Then I did one of the Minuteman quests that he gave me, after setting up the radio station at the Castle... and afterwards he suggested that we should get some men together and go take the Castle. Again.

"Seriously?" I said. Luckily, "I've already cleared it out" was one of my reply options... to which he responded in amazement, as if he hadn't been there at the time. Now my mission is to go back to the Castle, again. But Preston is still hanging out with me, fighting and looting alongside me and my "normal" companion. I bet I could keep this up for the entire game, as long as I don't complete the Castle quest...

Is this a hidden cheat for those who want to have 3 in their party? Are there other quests that behave this way (i.e., get one step from the end, then walk away from the mission and take both companions with you)? Or is this a unique glitch?

had the same thing happen to me. I took Curie with me when I took the Castle. During the events of the quest I dismissed Curie and took on Preston so I wouldn't have to chase him down. He has a tendency to head back to SH before taking the Castle completes. Well I got the completion for the Castle and located Curie near the door to the Castle. She had somehow wandered down to the water and was standing on the nearby boat. So I talked to her, using her invite dialog to dismiss Preston for me. Preston dismissed but then remained with me.

We took care of about 3-4 MM quests across the CW with Preston constantly by our side. His dialog had the invite to follow me, or trade with me. And he would not crouch when I would to go into sneak mode like Curie.

He eventually dismissed himself when Bonnie Shaw showed up at the Castle. That is when he stopped following us and went to tending some corn I planted.