There are several synonyms for the phrase "slip of the pen," which is commonly used to describe an inadvertent mistake made in writing. One such synonym is "typographical error," which refers specifically to errors made in the process of typing or printing. Another similar phrase is "writing error," which encompasses a broader range of mistakes made in the act of writing, including spelling and grammatical errors. Other possible synonyms for "slip of the pen" include "inadvertent mistake," "careless error," or simply "mistake." Regardless of the specific term used, slips of the pen can be frustrating but are a natural part of the creative process.