While reading last week's parsha, I noticed that the letter "nun" immediately preceding the verse Vayehi B'nso'ah ha'aron Bamidbar 10:35 was written as regular nun (tagin on top) rather than upside down, as the majority of Sifrei Torah seem to have this. I base myself in this on my own experience as a Ba'al Kri'ah over 30 years and having seen about 200 different sifrei Torah in that time that ALL have them. (See Talmud Shabbat 115b - 116a, for the source of the symbols, though it's not apparent that the format is specifically a nun, there.) The nun after verse 36 was upside down.
Does this make the Torah passul? I am surmising that it might, because one might argue that this is like an extra letter written in the Torah, which should not be there. Incidentally, there was a space between the 1st nun and the preceding as well as the next parsha, so it did not look like it was attached to any word.