It doesn't really matter. I like calligraphy very much and often make careless mistakes, but our teacher says it doesn't matter, because calligraphy mainly depends on how the words are written rather than the content, or no one cares too much about the content at all. If you make a mistake, others may not find it, and even if you do, you won't care too much, because it is a calligraphy work, not a copy of Chinese or something.
If it's really important, write a new one. Write carefully. It won't be long.