Mistakes in writing

Hi Gelever
Spelling errors come under Lexical Resource, so a Band 6, for example, would be "makes some errors in spelling and/or word formation, but they do not impede communication", while a Band 8 would be "produces rare errors in spelling and/or word formation".
Grammatical errors come under Grammatical Range and Accuracy and the score is split between the range of grammatical structures (from the simple to the more complex, such as a sentence beginning with a participle phrase or a conditional sentence) and the actual errors - including punctuation - in the sentence (such as using a singular subject and plural verb). In this set of descriptors, a Band 6 is "uses a mix of simple and complex sentence forms and makes some errors in grammar and punctuation but they rarely reduce communication" while a Band 8 is "uses a wide range of structures, the majority of sentences are error-free and makes only very occasional errors or inappropriacies".
In short, no, a single error in either spelling or grammar cannot reduce your band score from 9 down to 5.
Kind regards
Teacher Jill