I have been asked this question so many times by my IELTS students that I feel the need to make this clear to those who are worried about how your test is viewed. Technically, losing marks only happen when you make spelling mistakes or provide incorrect answers in the Listening and Reading tests. For instance, if you have produced 35 accurate answers but have spelling errors in three of them, then you will lose 3 marks and obtain only 32 out of 40, which will lead to a reduced band score.
In the IELTS Writing or Speaking modules, however, scores are chosen. Your written or spoken performance will determine if the assessor will be persuaded to give you a higher score or otherwise. Marks are not given and marks are not cut. If you have made errors, chances are that you will not be entitled to a higher band. The examiner will assess your performance according to the four criteria and select a band that would best describe your language ability in each criterion. The only case where scores are deducted is when the examiner feels that your writing is severely under length in the IELTS Writing task.
I hope that answers some of your questions. Let me know if there is anything else on your mind and I will be happy to respond to them here. For my current IELTS students, see you in our next lesson!