My main pet peeve is still the apostrophe-s for plurals, but I do have a new secondary pet peeve: using "their" when referring to one person.

For example, "A writer should always check their grammar." In this case, "writer" is singular, but "their" is plural. This should read, "A writer should always check his [or: his or her] grammar." To avoid the "his or her" without using the masculine "his," this sentence could be written as, "Writers should always check their grammar."

These mistakes are pretty easy to spot, though they do confuse some folks when using such words as "everyone" and "each," which are both singular.

For example, "Everyone loves to read their own work." In this case, "everyone" is singular, but "their" is plural. This could be corrected using "his or her" or by changing "Everyone" to "people."

Any discussion on this? Other examples? What's your pet peeve?