The antonyms for the phrase "free error" can vary depending on the context of the sentence. If you're referring to a mistake that was not "free," you could use words like "costly," "expensive," or "damaging." Alternatively, if you mean the opposite of an error, you might consider words like "accuracy," "precision," or "perfection." Other antonyms that could be applicable include "correctness," "efficiency," or "reliability." It's important to consider the intended meaning of the phrase and the context in which it will be used to choose the best antonym for "free error.