A Sefer Torah, tefilin, or mezuza with any missing or extra letters is invalid and may not be used. A megila or navi with missing or extra letters may be used bidi’eved.1
Extra letters can always be erased – even in tefilin and mezuzos which have the requirement of kisidran. If the extra letter is in the middle of a word, the empty space left after it is erased must be filled in (by lengthening or thickening the letters on either side) to ensure that it doesn’t now look like two different words. If the extra letters are at the beginning or end of a word, or between words, the empty space might need to be filled, but only if it would otherwise leave the space of a parsha.
Missing letters in tefilin and mezuzos can not be fixed, unless the error is noticed and corrected before the next occurrence of one of the names of G‑d (which can never be erased). In situations where a correction is possible, all writing done after the missing letter will need to be erased and rewritten kisidran.2
Missing letters in a Sefer Torah, megila or navi can usually be filled in by erasing a few letters or words on either side of the missing letter and rewriting them a bit narrower to make room for the additional text.
If it is not possible (or not practical) to erase the letters around the mistake, it is also permissible to add in the missing letters or words between the lines. This practice was fairly common in older Sifrei Torah, but it is almost never done today. Another possible solution that was once widely practiced but is now rarely used is to cut out the particular letter or word (sometimes along with adjacent letters or words), leaving a hole in the parchment. A scrap piece of parchment would then be glued onto the back of the yeri’ah to patch the hole, and the missing text would be written onto the patch.3
At the present time, writing between the lines and patching holes are no longer considered appropriate under most circumstances. That being the case, if the missing letters can not be seamlessly added into the regular line of text, the whole yeri’ah of klaf is removed from the Sefer Torah and a new yeri’ah is written to take its place.
Whenever text is erased in a Sefer Torah, navi, megila or mezuza, care must be taken to ensure that the sirtut is still clearly visible. If the sirtut is rubbed out or faded, it must be replaced before any new writing is done.
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The same would apply to transposed letters – both where the wrong letter was written in the place of the right one, and where the right letters are in the wrong sequence. ↩
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As noted in Purchasing STAM: part 8, there is a disagreement among the authorities regarding exactly how the subsequent writing should be erased: It may be enough to erase most of every letter so that all the letters are lacking in their ikar tzura.↩
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It is highly questionable if a Sefer Torah with a patch could be considered “mehudar”.↩