I.D. 10-T or ID10T is an alphanumeric indicator for the word "idiot." Depending on how this text element is set up, it can be more or less obvious that the numbers and letters are supposed to stand for something visually.
This slang term is used by technical support staff in IT departments to refer to an error caused by the user, rather than an actual problem with the equipment. For example, "the caller on line 1 has an I.D. 10-T error." The term is used to avoid being blatantly offensive.