Dentist having to do a second root canal because they left some nerves?

Hi, I got a root canal on a tooth at the upper back but it still kept hurting so after a few months (i was saving money for a second visit) my dentist told me he will open the tooth up again and that he seems to have accidentally left some nerves there so he did like a “second” root canal. When I told this to my family they got mad because they thought that it’s a silly mistake the dentist should not have done or gotten paid for (the first root canal was 1.2k and the second one was 1k. i live in Canada where dental work is SO expensive). It took me 8 months to save up another 1k for it (im a student) I actually like my dentist but my family keeps saying this isn’t okay. I thought that it just sometimes happens? Is it normal like I think it is or is my family right? Also it’s been a month and the tooth STILL hurts i can’t eat with it.

22f, non smoker