slip of the pen
An unintentional mistake in one's writing, such as incorrect spelling, word choice, or meaning. Whoops, I meant to tell my mother that she was welcome to call over next Monday, but I actually wrote "next month" instead. Oh well, she'll understand that it was just a slip of the pen. Now take your time while writing these test essays—you don't want a few slips of the pen to be the reason you get a lower grade!