More is less. A very common mistake is to answer in more words than instructed. If the task says "Not more than 3 words", answering in 4 or more words will definitely cost marks.
Less is less. The length of a written task is crucial. When instructions mention a minimal number of words (250 for essay, 150 for report or letter), it means that any work shorter than required will be penalized.
Longer essay doesn't mean better mark. Another common misconception is that longer essays score better in IELTS. Not only is this a myth, but also a dangerous one. Writing a long essay can indirectly cost marks, because the chances of making mistakes increase with the number of words and sentences.
Changing the subject is unacceptable. Every so often a student is asked to write on topic, that he doesn't understand. To avoid the disaster of missing a whole task they decide to write on a slightly – or entirely – different topic. The sad fact is that no matter how beautiful the submitted work is, the wrong topic means zero score. Another similar pitfall is to omit parts of the given topic or ignore the guidelines in your work. Every point the topic refers to needs to be covered because the examiners will be actually counting them.
Good memory can get you in trouble. Having seen that the topics sometimes repeat, "smart" students with good memory decide to memorize essays. This is a terrible mistake to make because the examiners are trained to look for memorized essays and have firm instructions to disqualify such works on the spot.
Accent is not important. Pronunciation is.! IELTS, being a test for non-native English speakers can't penalize people for having an accent. The problem here is that not everyone knows the difference between speaking with an accent and mispronouncing the words. No matter how strong of an accent a person has, the words are to be pronounced correctly or it will cost marks.
It is not the ideas that are important, but the way they are described in. Many students think that expressing the wrong ideas (whether it is in essay, letter or discussion) can harm their score. The truth is that no idea can be wrong and the ideas are not important on their own, it is the way they are expressed in that important.
Connective words: the more is not always the better. Smart students know that one of the essay marking criteria are coherence and cohesion, and what better way is there to demonstrate cohesion than to use lots of connective words, right? Wrong. Overuse of connective words is a know problem, which is easily recognized and penalized by the examiners.