A typographical or grammatical error in any type of electronic correspondence that you send to others (normally at work). The error can be subtle or obvious but it's just like the Waldo books where you have to search, sometimes for a very long time, in order to find the error.
Here is an obvious example:
I can't believe that your leaving the company on such bad terms.
In this case, "your" should actually be "you're" to be grammatically correct.