Hi,
When my shift was over today, I did a little shopping before going home. When I got to the register, after waiting in line for a bit, I noticed that the lines were getting quite longer, all the way to the aisle behind us, and there were only two cashiers. I looked at the cashier in front of my line and see that she is moving really slow for such a long line, and seems to be having a decent conversation with the customer. I don’t mind her talking, but her speed compared to mine was as if she is moving in slow motion. I checked back at the two lines, and see that they are getting too long for just two of them to handle. I decided to do them a favor, by calling "X girl’ to the front and open up. The cashier in front was surprised, because I was supposed to be leaving. She told me that I wasn’t supposed to do that, and that X was going on break.
I really didn’t know that because when I was getting to leave 15 minutes earlier, X was already getting to go on break. So, in my guess, X was supposed to be back at that time. Anyway, X came along with the manager, and I asked her if she was still on break. She said yes, but she’s opening up now. The cashier said, half jokingly and half seriously, that I should have more respect for my coworkers. The manager also said that I could have called "Y girl’ instead.
Perhaps there’s a rule that I am not allowed to use the microphone to call someone like that if I am not on duty? Was it better if I just asked that cashier to call for help, instead of doing it myself? I was just trying to do them a favor, yet, seems like it backfired in the end. They were all sorta chilled, though, so I don’t know if it was seriously offensive or not.
By the way, with all these work stories, I think I should get a reddit account. 
As someone who works two jobs as a cashier, yeah, to me, you were being rude and inconsiderate to your coworkers. I understand having that coworker who is slow, but once you clock out, you’re a customer. In both my stores, you’re not allowed behind the cash registers or touch anything work related once you’re off duty. It’s a Loss Prevention thing.
Also, how long are your breaks? Chances are, the time it takes you to get your stuff and clock out is not the length of a break. Always work on the assumption that said coworker is still on break to avoid confusion. Next time just ask the cashier to call for back up. Just don’t be surprised if you get glares for not being understanding/patient due to the long line. Your coworkers know when to call for back up and I don’t know, but there may or may not be a rule for how many people there need to be on a line in order for them to call for help in your store. Although, it sounds like from your manager’s comment that it was okay to do that, just that you shouldn’t be doing it, since you were off duty
This is probably your first job, so you’re still learning things, which is fine. However, it’s better if you empathize with your coworkers and be considerate of them instead of trying to figure out how to help yourself in situations like this one. It will help you in long run and will make you look better. Just my two cents. (pun intended)
I used to clock out once and still helped a young girl ring up alcohol. I doubt it’s that big of a deal to help out my coworker a bit. The general rule is that if you have more than 4 or 5 customers, call for help. The number of customers lining up was twice that. It’s not that I was impatience, but I thought the line’s cashier was too busy having the conversation to notice, and I just wanted to help them out, not myself since I was the next person in line. Funny enough, that same cashier used to called me once right after I came back from break, before I was clocking in, and I never complained about that.
The next day after that, the X girl talked to me in private that I shouldn’t mind what happened. That cashier changes her attitude quite suddenly, and that I shouldn’t worry about it. I told her that I did that only because the line was really long, and that she understood. That was all. Nothing else was ever mentioned about it, and we got through the day as chilled as normal.
I think to say that I was rude is a little exaggerated. But yeah, it’s my first job, but I do know how things work in the store. And we are on good terms, otherwise, I wouldn’t do such thing if I was calling someone else (Y guy or Z girl). But anyway, next time, I’ll let the line’s cashier know instead of doing it myself. 
As someone whose worked for quite some time, I’d like to give you some helpful advice 1. Know your role and position and that will guide you in how to handle a situation. 2 If you are off the clock, you should not be assisting coworkers . If you are injured, you are not covered by workers comp in most cases. There is a lot of info to learn from your first job, it will help you in the future if you remember it for future reference.