In order for a Mezuzah to be Kosher, it has to meet thousands of requirements. Not only does the text have to be accurate, but every letter has many layers of rules that determine if it is Kosher. Since every Mezuzah is written by hand, few are exactly perfect. This doesn’t always mean that they are not Kosher though. Often when a letter is not written in the ideal way, it is still Kosher after the fact. An expert Sofer (scribe) has to be very well versed in all of these rules. He has to be able to make hundreds of ‘rulings’ per day about all kinds of letters that are written in a less than perfect way. In addition to this, the Sofer has to keep a sharp eye on letters that may have fine cracks to repair or may have a hairline connection between them which needs to be separated.

The problem that arises is that while the Sofer is busy looking at all of these details, it is entirely possible that an entire word could be missing and he will overlook it. Human nature is such that it is quite difficult to concentrate on fine details at the same time as you are looking at the big picture.

Throughout the ages, Sofrim who were very meticulous would check there work at least two or three times. They would check once for spelling and then again for the finer details. Others would have a non-Sofer check the spelling, since they will not be distracted by the finer details. Nevertheless, it was not uncommon to find a Mezuzah or pair of Tefillin with a spelling mistake, or even an entire word missing or repeated.

Nowadays it is very rare to find any spelling mistake in Mezuzahs. This is because of technology that was developed in the ’80s. It is a software program that can check Mezuzahs for textual accuracy. The Mezuzah is scanned into the computer. The software actually learns the writing of the particular scribe and then checks the Mezuzah. If the computer finds anything that it percieves as an error, a window pops up with the section in question highlighted. The person using the software has to make a determination at that time if it is a mistake or not.

When this software first came out, there were a lot of Rabbis who were afraid that people would rely on the computer to do the Sofer’s job. This would be unacceptable because in Judaism, all Halachic questions have to be decided by a Torah scholar, not by a computer. Other Rabbis were very much in favor of the new development, saying that since no person is perfect, anything that helps to prevent mistakes is a welcomed tool. They stressed though that the Mezuzahs still needed to be checked by a certified scribe in addition to the computer’s check.

Today, this computer checking has become the industry standard and it is very rare to find a Mezuzah on the market which has not undergone this process. Because of this, it is very rare today to find and actual textual error in Tefillin and Mezuzahs.

The great thing about the computer check, is that you only have to do it once. After that, when you get your Mezuzahs checked periodically, it needs to be done by a human Sofer. A lot of people ask me if I can computer check their Mezuzahs for them. I usually tell my customers that if they purchased their Mezuzahs from a competent source in the last 15-20 years, they probably don’t need to bother.

Mezuzah Checking.
Mezuzahs must be checked every 3.5 years to ensure that they are Kosher. Many Rabbis have reccomended that they be checked every year. There are many things that the Sofer will look for when he checks your Mezuzahs. The two basic categories are: Damage that may have occurred do to water, heat etc. and problems in the way the Mezuzah was written such as spelling mistakes, touching letters, improper spacing etc.
If a problem is found, the Sofer has to determine if it can be corrected or not. The laws regarding this are very complex. Here are some of the basic concepts involved:

Shelo Kesidran – Writing in order.
Halacha dictates that both Mezuzahs and Tefillin must be written in chronological order from beginning to end. This means that the second letter must be written after the first and the third after the second etc. If the Sofer discovers a mistake in the first letter after he has already written the second letter, he must first erase the second letter before he can correct the second.
This only applies if the mistake is one which causes the letter to be unrecognizable to a young child. But if the letter is obviously recognizable but only has a technical flaw, it may be corrected even after other letters have been written.

Chok Tochos – Scraping.
The Torah says, “you shall inscribe them.” The Rabbis say that this means that you must write the letters but not “carve” them. Practically, this means that if one writes a flawed letter, he may correct it by adding but not by scraping.

Tefillin Checking.
Checking Tefillin involves the same rules as checking Mezuzahs. It also requires the Sofer to check the Batim and Retzuos, to insure that they are Kosher. Unfortunately, many sofrim only check the scrolls but not the other parts of the Tefillin.
Tefillin checking tends to be much more expensive than Mezuzah checking because the Sofer has to re-assemble and sew shut the Tefillin. This is a time consuming process.