Skipper Maguire and Nemanja Matic, two of United's most senior players, sat down to discuss ways in which players would be disciplined if their conduct fell below expected standards.
The pair came up with a series of regulations for the squad to follow and if somebody did fall foul, they would be forced to pay a fine.
But incredibly, Maguire ended up being one of the first players to be hit with a fine after Matic caught him on his phone while in the gym.
"He was fined, yeah,' Matic told Manchester United's official club website.
"He brought his phone to the gym. The problem was [that] me and him, we decide what are the fines! A few days later, he brought the phone to the gym, so I said, 'Harry, I'm sorry...'
"It's very important, discipline, in our team, so that's why we made some basic rules.
"Not to be late for training, not to be late for meetings, no phones in the gym. The guys struggled to adapt for a few months, so we had a lot of money in our account, but it's going to be better!"
Maguire, however, reckons the fine being dished out was not in the "clear and obvious" section and might have needed a VAR check.
"I've been fined once which is still debatable," the England international explained.
"It's crazy, but I let Nemanja do it. I took the hit and put the money in the pot. When you meet in the gym before training for pre-activation, you are not allowed your phone. We did pre-activation; I didn't have my phone.
"But then you go outside to put your boots on and you aren't allowed to go on your phone in that period. I had my phone and carried it out with me.
"[But] I wasn't on my phone. Straight after training, I was going to put my trainers back on and go up for a massage instead of coming all the way back [to the changing room]. I could have gone straight to a massage, and Nemanja fined me for that. But he's strict with everyone so I'm happy. I can set the standard."
Matic, the assistant to the fine manager has also had to fine himself after he was late for a team meeting prior to a game against Newcastle.
He recalled: "I was late to the meeting. In front of the room [where the meeting was taking place] I was talking to Paul [Pogba], so I will ask him to pay 50 per cent of my fine. I don't know if he will accept!"
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