Mezuzah Checking and Tefillin 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.
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.