My greatest pet peeve is the use of apostrophe S where no apostrophe goes. We learned in what - kindergarten? - that to make a word plural in the English language, you add an -s to the word (or -es when the word ends with s.)
There I was, surfing through the Reds products on MLB.com in search of Christmas presents, when bam! There it was. On a commercial website. A big, fat apostrophe on a welcome mat.