The sofer (scribe) must be very careful when writing the letters of a Sefer Torah, tefillin or mezuzot, especially when writing the letter vav. If the vav is written incorrectly, it may look like a different letter. For example, if the vav is not long enough, it may look like a yud. Alternatively, if the vav is too long, it may look like a nun-sofeet. Therefore, it is crucial that the sofer be very careful that the top, and and bottom of the nun are properly shaped and proportioned.
What if the vav looks like a letter yud, or a nun-sofeet? In both cases, the letter is pasul. If the letter is written in a Sefer Torah, then the Sofer must erase the whole letter and rewrite the vav.
However, while this solution is valid and proper when writing a Sefer Torah, tefillin and mezuzot must be written in order. This principle is known as kisidran. Therefore, in these cases, i.e., tefillin and mezuzot, it would not help to add ink or cut part of the letter, because the letter is considered to be written out of order, and the entire portion is invalid.
Sometimes, there is a doubt, and it is not clear whether the letter is too long, and looks like a nun-sofeet, or too short, and looks like a vav. In this case, the halacha teaches that one should bring a child, who is not a hacham and not a tipesh, and ask him to identify the letter. If he says the letter looks like a vav, we validate the Sefer Torah. If he identifies the letter as a nun-sofeet, we disqualify the letter. This is only done if the identity of the letter is unclear. If, however, the letter is clearly too short or too long, the latter is pasul.
Summary: When a sofer writes a vav in a Sefer Torah which is too long, and looks like a nun-sofeet, or too short, and looks like a yud. It may be fixed. If, however, he makes this mistake while writing tefillin or mezuzot, the letter cannot be fixed and the portion is pasul. If there is a doubt, a child is asked to identify the letter, and based upon his statement we determine whether the letter is valid or pasul.
