Inverted Nuns
In fact, it is far from clear whether one should do this at all. Most notably, R. Shlomo Luria (Maharshal) argued that the Talmudic passage this custom is based upon only mandates the typical break for a parsha and not any upside down or otherwise letters. The passage only states that a sign should be made for this parsha and nothing more. He argues that such letters in the Torah render the Torah passul (unfit for use). R. Luria also notes the lack of uniformity in presenting such nuns, there are 19 different ways he came across to make the nuns. Some even flip the nuns of the text of the Torah and do not place the strange letters prior to and after the parsha in question. Thus, according to R. Luria, all of our Torahs which contain such nuns are passul.
R. Yechezkial Landau (Noda B’Yehuda), however, among others, defends the custom. He claims that the use of such a non-letter i.e. an upside down or z shaped non-letter is the key to allowing such a practice. As since this is not a letter at all therefore it is just a sploch of ink which doesn’t render the torah unfit for use.
Although the nuns in last weeks reading are almost universal, there is another inverted nun in the Torah that is attested to by R. Shlomo Yitzhaki (Rashi) which, it seems, is not accepted at all. Rashi at the end of Parshat Noach says that the name of Abraham’s father, Haran has an inverted nun. But this doesn’t appear at all. (Another missing nun as it was.)
For more on this topic see here and here. Read She’alot u’Teshuvot Maharshal, no. 73; She’alot u’Teshuvot Mahram m’Lublin, no 75; She’alot u’Teshuvot Noda B’Yehuda, vol. 1 yoreh Deah no. 73; R. Menachem Mendel Kasher, Torah Shelmah, vol. 29 p. 124-130 (where he provided pictures of the various methods of writing the nuns); C.D. Ginsburg, Introduction to the Massoretico Critical Edition of the Hebrew Bible p. 341; Shnayer Z. Leiman, “The Inverted Nuns at Numbers 10:35-36 and the Book of Eldad and Medad” in Journal of Biblical Literature 93:3 (Sept. 1974): 348-55; Saul Lieberman, Hellenism in Jewish Palestine, 38-43; Emanuel Tov, Textual Criticism of the Hebrew Bible p. 54-55.