The phrase "slip the pen" refers to the action of making an error or mistake while writing something down. There are several synonyms that can be used to express this notion, including "misspell," "miswrite," "mistake," and "mess up." "Misspell" refers specifically to an error in spelling, whereas "miswrite" is more general and can refer to a variety of mistakes made while writing. "Mistake" is a broad term that could encompass any type of error, including an error made while writing. Finally, "mess up" is a more informal synonym that could be used to describe any type of mistake or error, including writing errors.