Bad fics... usually you can tell from the start they are going to be bad, so you are not disappointed (and you stop reading). Dumb-sounding summary, bad grammar, bad characterization, atrocious dialogue, sparse descriptions or simply an awful plot are alarm bells you can spot so soon like in the first chapter.
Exactly, when I see from the start a fic is bad or has really bad reviews or the plot doesn't quite appeal to me I don't read it. No hard feelings, I just skip it and try to find a fic I like.
I've read a few stories where the beggining was excellent, the writing was very good, all in all I liked everything about it. And then it basically took one sentence to make me hate it. e.g. Naruto starts sleeping around, you realize Sakura was still in love with Sasuke, some obnoxious oc appears and get so much development when I really don't want to know about him, someone dying without warning... I've used exaples from the Naruto fandom because they're the most fresh in my mind but they're not even the worst I've read.