To fulfill the mitzva of reading the Megilla, one must read it from a kosher megilla that was written in ink on parchment. If one recites it by heart or reads it from a printed book, he has not discharged his obligation (sa 690:3). If one reads most of the Megilla from a kosher scroll and the rest from a book or from memory, he has fulfilled his obligation, as long as he recites the entire Megilla without missing a word.
However, if the reader skips a word, or makes a mistake in one of the words that changes the meaning of the word, he has not discharged his obligation according to most poskim, and he must read the Megilla again properly.
Even when the reader reads all the words properly, a listener does not discharge his obligation if he fails to hear one of the words. This is the most important practical halakha of Megilla reading, because people sometimes fail to hear all of the words, due to the excess noise caused by the presence of children in the synagogue. This problem becomes especially acute after Haman’s name is read, as the reader sometimes continues reading before the children finish making noise, causing those who sit in the back of the synagogue to miss a word, thus forfeiting the entire reading.
If one missed a word while hearing the Megilla reading, the solution is to read the missed word or words immediately from the printed Megilla one is using. If, in the meantime, the reader continues to read ahead, one should continue reading until he catches up with the reader. Even though the printed book in front of him is not a kosher Megilla, he may use it to fill in the missing words, be-di’avad, since he hears most of the Megilla from a kosher scroll. However, when listening to the Megilla from the reader, one should not read along with him from a printed version.
In my humble opinion, if the reader made a mistake that affects the meaning of the verse but most people do not understand the difference in meaning, he does not have to read the word again, be-di’avad, because the listeners did not misunderstand the meaning.
Some have a custom to read certain words twice because their correct reading is uncertain. For example: ke-omram/be-omram (Esther 3:4); laharog/ve-laharog (ibid. 8:11); bi-fneihem/li-fneihem (ibid. 9:2). One fulfills the mitzva even without following this practice.
. Those who go above and beyond purchase a kosher megilla, from which they can read in order to fill in any words they may miss. This way, they fulfill the mitzva in the best possible manner (mb 689:19). However, if one does not know how to pronounce the words properly, he does more harm than good by reading from his own megilla. After all, we already learned that a mistake that affects the meaning of a word invalidates the reading. sa 690:4 and mb ad loc. 13 explain why a listener should not read along with the reader from a printed megilla.