The Halacha states that a mezuzah should be checked twice in seven years (Shulchan Aruch, Yoreh Deah 291:1). By phrasing it like that, there is no requirement to check on a very specific date, only that the mezuzah should be checked regularly. Sometimes water damage or cracking occurs, and these are discovered when the mezuzah is checked. If such problems are discovered the mezuzah would usually have to be replaced.
Additionally, before a mezuzah is ready to be affixed initially, it has to be reviewed by a sofer to make sure that it was written without mistakes. In fact, many people prefer to have the mezuzah checked by computer as well, to ensure that all the words/ letters are there. Here we display two mezuzos which are invalid, one because of a water damage smudge, and the other because it was missing a word when it was originally written.