1. Informal contractions like don't. shouldn't, won't, isn't.
Remember, you must use only formal style in your IELTS essays (even when writing a letter to a friend). Therefore, use only full negation forms: do not, should not, will not, is not.
2. Omitted points (full stops) in the end of sentences.
It's easy, isn't it? Just always end your sentence with a full stop and start the next one with a capital letter.
3. Too many commas in extremely long sentences.
If you notice that your sentence stretches across 4-5 lines of your essay - something definitely is going wrong. It is much better to write shorter, concise sentences without punctuation excess.
4. Wrong apostrophe usage.
Remember how possessive case is used - we put an apostrophe before "s" in singular (a singer's mike) and after "s" in plural nouns (singers' mikes).
5. Exclamation point!
Do you really need it in your IELTS essay? Think twice. Are you being formal when putting these exclamation points in the end of your sentences and adding this emotional tone. I doubt it. Leave it for your text messages.
6. Suspension points (three dots — ...).
Same as wit the exclamation points. These elision marks are used in novels and fiction books to express sadness or confusion. You definitely don't need this in your IELTS write-ups! Make sure to stay formal and take the style example from scientific reports.