some of you will find this amusing, some more than others...
At work I deal primarily with two groups of students, first-time freshmen, which we abbreviate FTF, and transfer students, which we abbreviate trans. Yesterday I was editing a document where someone had spelled all of these out on every reference, so I'd written the abbreviations above them. When I finished I realized that instead of "FTF & trans." I'd written "FTM & trans." Luckily I caught it before it got sent to my boss, though she would have probably just been confused, or not noticed at all.