By mistake is a commonly used phrase to describe an accident or error that was not intentional. However, there are several synonyms for this phrase that can add some variety to your writing or speech. Some alternatives include accidentally, unintentionally, inadvertently, by chance, haphazardly, randomly, unconsciously, and unexpectedly. Each of these words expresses the idea that something was done without deliberate intent or purpose. Choosing the right synonym for by mistake can help to clarify your message and convey a more precise meaning, depending on the context in which it is used.