Mistakes such as this are very rare on the IELTS, but if one happened, it would be quickly fixed (due to the advanced statistics they use, an incorrect (on their end) answer would show up very clearly). They would likely strike (delete) the question from the test.
Test makers are human beings too you know. On every standardized test you can find examples of question or answer errors of some kind.
In our case, our task is even more complex. We have to make sure three tests worth of answers are correct, in not one, but three places (tests, interactive tests, full course). This is almost 1000 places where a mistake could be made.
Rest assured, however, that no student would ever have their mark affected on the real IELTS by an examiner error.