Occasionally, due to graggers, sliding chairs or a sneeze, one may miss hearing a word or more of the Megilla. If this is not rectified immediately, it invalidate the entire reading, as the Megila must be heard in order and the missing segment cannot be made-up later. However, if one is quick on the draw, he may read the omitted words himself and quickly catch up to the Ba’al Koreh. Even if one does not own a Kosher Megilla, and the words read from a Chumash are as if they were said by heart which is generally not valid, for the Megilla we are lenient provided that a majority of the story was heard from a Kosher Megilla. Of course if the Ba’al Koreh reads too quickly to catch-up, this solution is not relevant. If this happens, and it happened to me once many years ago, there is no solution another than hearing the entire Megilla again from the beginning.
If one was daydreaming during the Kriya and did not pay attention to some of the words, the Poskim debate the necessity of hearing the Megilla again. The suggested method to prevent this problem is to read along, or at least follow, word-by-word with the Ba’al Koreh from a Kosher Megilla. This is one of the main reasons why many individuals strive to purchase their own Megilla, even though they will never read from it publicly.