Not leaving a sefer lying around without fixing it, and related matters
1. One may not leave a damaged sefer Torah (or any other sefer) more than thirty days, as it is said, “Let no evil dwell in your tent” (Job 11:14). It must be repaired or set aside. One may not fix anything in a sefer just because he thinks there is a mistake; it must be clearly proven that there is an error.
2. A sefer Torah with three mistakes may not be read from until it has been checked through, because it is assumed to be deficient. If one mistake is found and fixed, and two or three more are found and fixed meanwhile, the whole sefer does not need to be checked through unless there are three mistakes present simultaneously, and this is the custom (Benei Yonah according to Sifrei Zuta). However, if a great many mistakes were found, as many as 85, even though they were fixed as they were found, it is perhaps better to check the whole sefer (ibid.). If one knows it to have been checked by an expert proofreader, subsequent mistakes may be corrected as they are discovered, and the whole sefer need not be checked again, and this is the custom (ibid., in brief).
3. If a sefer Torah has up to three mistakes in each column, they may be repaired, but if four, it should be buried. If most of the letters are as they should be, and each column has at least four mistakes in it, the presence of one column without errors renders the rest permitted for correction. This applies only if it was written without mistakes originally; one may not swap out a sheet and insert an error-free sheet so as to permit the rest of the sefer to be corrected (Tosafot, Rosh).
4. All this applies if he wrote malei letters haser [omitted letters], since to superscribe the missing letters would degrade the sefer. If he wrote in extra letters, they may be erased even if there are a great many of them on every column, because erasing is not as degrading as superscribing. When he erases, he should widen the preceding and succeeding letters so that the gap won’t divide the word into two. See above, 11:9, on erasing the second word in a repetition.
5. If one accidentally left out a word, or more, he may superscribe them above the line, but may not put them into the space between columns.
6. If one accidentally skipped two or three lines, it is the custom to correct it (with no doubts as to the validity of this) by erasing several lines and making the writing smaller to fit in the missing part (see above, 14:4) [that is, making three lines over two erased lines, etc.]. But if he skipped more than three, he should not fix it like this, because it will be conspicuous; he must replace the sheet. If he transgressed and did it anyway, we cannot say that the sefer Torah is invalid (Sh-at Binyamin and other posekim).