Halacha (Jewish law), Chaf Alef (twenty-one), Maran (our master) writes. First, Maran tells us that for every parasha you write, you should reread it thoroughly three times. Then read it again before you actually place it in the compartment in the Tefillin (phylacteries), to make sure you put the right parasha in the right compartment. Of course, the parashas go in a certain order in the boxes of the Tefillin, so we want to make sure you don't place the wrong parasha in the wrong box.
Mishnah Berurah Kuf Dalet (one hundred four) says that if you find, let's say, a word missing a letter, the whole thing is pesulah (invalid). It is not only the word that is pesulah; the whole parasha is pesulah because the Tefillin has to be written in order. Once one word is pesulah, you can't fix it. Right?
Hacham David, in his sefer, writes that he shouldn't continue writing the next parasha, because the next parasha also doesn't have a pesulah. So he should continue writing the parashas in order, as written in the Torah. And if he wrote the next parasha after it—if, let's say, he wrote the next parasha—then if he said the next parasha before it, everything written after it is not allowed. And he just reads it after it becomes pesulah. Before he gives the parashas that are in the Tefillin, he will go back and read the whole parasha again, so that when he gives them all at home, they will see it and will not change one parasha for another.