1,403. Reading the Megillah Out of Order

Hilchos Megillah v’Chanukah 1:14

Pursuant to the previous halacha, if 14 Adar falls on Shabbos, the residents of unwalled cities read the megillah on Friday, while the residents of walled cities read it at their regular time, on Sunday. If 15 Adar falls on Shabbos, the residents of walled cities read the megillah on Friday, 14 Adar. The residents of unwalled cities also read on that day, which is the regular time for them. Therefore, when 15 Adar falls on Shabbos, everyone reads the megillah on 14 Adar.

Hilchos Megillah v’Chanukah 2:1

If a person reads the megillah out of order, he does not fulfill his obligation. If a person was reading, missed a verse, read the next verse, and then went back to read the verse that he missed, moving from there to a third verse, he does not fulfill his obligation because he read out of order. In such a case, he should begin start from the second verse - the one that he missed - and continue reading from there in the proper order.