Don't worry about the occasional mistake. The examiner will expect some mistakes – after all, English is a foreign language for you and people make mistakes in speaking foreign languages. The examiner is not making a note of every single mistake that you make. This would be impossible to do and concentrate on your speaking. He will more get a general impression of your English accuracy so individual errors don't matter. More important is your communication.
Perfection is not needed. You can still make some errors and get a 9 (not many errors though). So don't let making errors upset you. Get on with the talking and concentrate on your communication.
One thing that puts candidates off is that the Speaking test is recorded. This is done so that, if necessary, the speaking can be re-marked. If the bands for a candidate's writing and speaking for instance are very different (more than 2 band difference), then the candidate's test is re-checked.
If the speaking was not recorded, then this could not be done. This doesn't happen very often. Sample recordings are also sent to the IELTS administration to be monitored to make sure that examiners are doing a good job and assigning the correct bands. So, try and forget that the recorder is there and get on with answering the questions.