Listening has 40 questions, one mark each. The scale is steep at the top: 30 correct is band 7.0, 35 is 8.0. Five careless mistakes cost a whole band.
1. Spelling and plurals
In gap-fill questions, an answer you heard correctly but wrote incorrectly earns nothing. The usual suspects: "childrens", "recieve", and singular/plural slips ("two ticket" for "two tickets").
On a paper test you get 10 minutes to transfer answers. Spend them checking spelling and plurals, not guessing new answers.
2. The word limit
If the instruction says "Write ONE WORD AND/OR A NUMBER", two words is wrong even when the meaning is right. People write "a new library" when "library" was the answer.
3. Missing the correction
The recording changes its mind on purpose: "The meeting is on Tuesday... sorry, Wednesday". Write the first one down and move on, and the mark is gone. The signals are "actually", "sorry", "in fact", "but".
Read each question in advance and mark what kind of answer it needs — a number, a date, a name? Then your ear catches "Wednesday" when it arrives.