The problems with fan fictions

have you ever read a fan fiction and seen mistakes on the story line it's self? or wondered what would really if you added a Mary sue into some of it or the consequences of their actions, behaviors, or choices could bring or the love at first sight thing and it depends on the fandom also this story is not including lemons anything inappropriate is censored I will only include fandoms that I know though I will say this in some cases Writing fan fiction can be good practice for original books and in some cases not meany but some the oc actually works well with both plot and character s so those who don't like don't have to read thanks