There are several synonyms for the phrase "what a curious mistake." Some alternatives include "what an odd error," "what a strange blunder," "what a peculiar goof," and "what a bizarre slip-up." These phrases all express surprise or amusement at an unusual mistake. While they are all similar in meaning, there are subtle differences in connotation. "Odd" implies something unexpected but not necessarily out of place, while "peculiar" suggests something uniquely strange. "Bizarre" and "curious" both convey a sense of extreme unusualness. Using these synonyms can help to add variety and interest to your writing.