10. Back in the day. Originally, they did not have set haftaros to lain. Instead, each person chose a section of Navi related to the parsha or a seasonal topic (כסף משנה פי''ב תפילה הי''ב).
11. Nowadays. After time, communities chose set sections for each haftara related to the week’s parsha. The Gemara (מגילה דף ל''א ע''א) already mentions several set haftaros for Yomim Tovim. Different minhagim evolved among different communities.
12. Deviated from the minhag. There are certain times when it is accepted to lain special haftaros, e.g., for the Daled Parshiyos or for double parshiyos. If the regular haftara was mistakenly read instead of the special one, there is no need to lain the right haftara as the haftara lained was related to the parsha. However, if a haftara completely unrelated to the parsha was mistakenly lained, the tzibbur is not yotzei and they must lain the correct haftara with the brachos (שערי אפרים ש''ט סי' ל''א, שנות חיים שו''ת סת''ם, ה' קה''ת סי' י').
