How bad do you have to be to get fired from a part-time job?

I've just finished Grade 11 (last exam is tomorrow) and I've been working at Subway for about 10 months now, part-time. I feel like I've been doing a good job. I haven't ever been warned of getting fired or something, just when I first started, the owner would tell me, "Oh if you text alot, you'll get fired", "People have been fired because they talk to their friends alot", but more i'm sure just to warn me what I should not do since I was first starting. I'm usually pretty flexible with when I can work, I always try to work hard, I sometimes make mistakes but my owner just tells me if I have, and he even jokes with us sometimes. I wanted to know, how bad would I have to be to get fired? I have an issue this week with work because my dad is on vacation this week and my mom works so I'm stuck watching my sister, plus exams, so I've had to negotiate a bunch with my owner with when I can work this week, because he's given me a couple shifts I cannot work because of my sister. I am afraid something like that will annoy him and cause him to think I am not flexible and want to fire me. But he doesn't seem like he is mad, he's trying to help find a day where i can work. And I make mistakes sometimes, but I've asked one of my co-workers who talks to him alot and apparently he hasn't said anything bad about me, when he does about others. What things/ mistakes would make a boss want to fire you from a place like Subway? So I can avoid them :) Thank you!