Ok, I want to know if someone actually told you that you had to stay, or if you just felt you had to, that determines whether it's a FML or a YDI. Anyway, assuming you were forced, pretty sure all of this is illegal 1) you're not being paid and 2) I know for my country anyway, they're not allowed to make you work over a certain amount of hours. Also, why is the person who didn't finish THEIR work not there with you helping finish it? Anyway, when I worked this type if job, I find the best way to deal with this stuff is put your foot down, don't be a pushover because people will walk all over you. Someone doesn't finish their work, that's their problem, not yours. Someone asks you to stay after hours, especially unpaid, tell them no, you have your scheduled work day and that's all the tone you'll spend there. I find people are sometimes so worried about their job that they fail to realize they have rights and don't need to put up with bullshit.

it's time revenge... maybe do that yourself

Wow don't know about you but it's illegal to work over a certain time frame where I come from. I thought that would be pretty global.. You really need to stand up for yourself now while it's early.. It gets much tougher later

Where I'm from its limit illegal to work over a certain amount of time. However you should be getting paid for it. Esp if it's counts as overtime. Also, tell them no. The other persons job is not yours. You only need to do your work.

If you're an hourly employee (& it sounds like you are) you have to be paid for that time if they asked/told you to stay. That's the law.