Related for Menachot 58:17
אגרא חמוה דרבי אבא הוה ליה יתירות בסיפריה אתא לקמיה דרבי אבא א"ל לא אמרן אלא בחסירות
R'Isaac B'Samuel B'Martha said in the name of Rab, provided only the scroll was for the most part written correctly. Abaye asked R'Joseph, How is it if in that column there were three mistakes?<span class="x" onmousemove="('comment',' Is this column to be regarded as free from mistakes since the three mistakes in it may be corrected?');"><sup>25</sup></span> - He replied, Since it is permitted to correct them they are regarded as already corrected. This rule<span class="x" onmousemove="('comment',' That a scroll with four mistakes in each column must be hidden away.');"><sup>26</sup></span> applies only when letters are missing, but when there are too many letters it does not matter.<span class="x" onmousemove="('comment',' Since the additional letters can easily be erased.');"><sup>27</sup></span> And why is it not so when letters are missing?<span class="x" onmousemove="('comment',' The missing letters can surely be inserted.');"><sup>28</sup></span> - R'Kahana answered, Because it would look speckled.<span class="x" onmousemove="('comment',' The insertion of missing letters above the lines would make the whole look irregular.');"><sup>29</sup></span> Agra, the father-in-law of R'Abba, had a scroll in which there were additional letters, so he came to R'Abba who told him the law: This rule applies only when letters are missing,