The phrase “lapsus calami” is Latin for “a slip of the pen.” Basically, it's the written equivalent of a slip of the tongue, an error which can be humorous, unintentionally revealing, or awkward, depending on the circumstances and the slip. A lapsus calami may take the form of a spelling or grammar error, a spoonerism, in which letters are switched around, or the replacement of an entire word or phrase. Even in the era of the spell checker, slips of the pen (or keyboard) can still occur, because spellcheckers will not pick up word substitutions, doubled words, or homonyms.
This term has its origins in zoology and taxonomy, where people often misspell scientific names, especially when they have not studied Latin. Although Latin was once the language of choice for educated individuals, making the use of Latin for scientific names very sensible, this language is much less widely studied today. This can make researchers prone to errors when they try to recall the spelling of a scientific name.
Zoologists actually have a specific name for a lapsus calami involving a scientific name; it's known as a nomen nullum, a “null name.” The spelling of scientific names can be further complicated by disputes over the scientific name of an organism, especially when a scientific name is derived from the name of the person who discovered the organism, or the location where it was discovered.
Sometimes, a lapsus calami takes the form of a Freudian slip, in which thoughts from the subconscious mind rise to the surface and express themselves on paper. In these situations, the resulting error can sometimes by humiliating for the person who makes it, especially if he or she has been obsessing about a specific person or action and this obsession worms its way into a written communication.
We all make slips of the pen now and then, especially when we are working quickly, or when we are distracted, splitting our focus from our work. While such errors cannot always be avoided, they are a reminder that it is a very good idea to read written material over carefully before handing it off to someone else. Sometimes a quick review will pick up a glaring mistake. If the document is especially important, you might want to ask a friend to glance at it as well, because sometimes we gloss over a lapsus calami or two when reviewing written work.