The pressure and stresses of exams can get to us all but keep calm, carry on and whatever you do avoid these surprisingly frequent silly mistakes.
Ignore the question
Everyone says it and it's true: The biggest exam mistake that is made is not reading the question. Start your exam by going over the rubric on the front to see what exactly you need to do (it may be different to past papers) and read every question at very least twice.
Figure out what exactly the question IS asking of you rather than what you WANT it be asking you.
Forget about the units
Units are important so be careful not to mix them up: Watch out for nasty questions which may involve different units. Convert any measurements - whether they be speed, distance, time or something else - into the same units before doing any calculations.
Also make sure to see whether or not the question wants your answer in a particular unit, these are easy marks to grab that so many miss!
Not bothering to show your working
Where applicable, always show your working and thought process. Even at University level exams, showing you understand how to answer the question will get you some masks even if you're unable to arrive at the correct result.
Forget how to use your equipment
You don't want to waste time trying to work out how to use your calculator or any other equipment (compasses can be very fiddly things!). This is especially important in subjects such as Maths or Physics where knowing how to setup your equipment - such as selecting between Degree and Radian mode - is needed to get the correct answer.
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Skip the planning and rush an answer
This is for those longer essay style questions that in some papers may be worth half or even more of the overall marks for the exam. When it comes to these longer responses, take your time and plan out your comments.
Don't focus on quantity but rather quality, making sure to actually answer the question as opposed to simply panicking and regurgitating everything and anything you remember from class as it leaves your brain via your pen.
Leave too little time to finish
While you should take your time to answer the questions, you don't want to spend so long writing that you don't have time to actually finish the paper. Use the mark scheme and time allowed as a guide for how long to spend on each question: As a trivial example. a 60 mark paper with a one limit means you should be looking to spend about a minute on each mark, so a 5 mark question shouldn't take you a quarter of an hour to complete! If you get stuck, move on to the next question and come back later if you have any time left over, there may be easier and quicker marks for you to gain.
Don't check your answers
Always go back check your answers not only after you've finished the exam but as you complete questions too. Pay attention to spelling, punctuation and grammar; even if you won't be specifically marked on it, it'll help the examiner understand you!
In mathematics-type questions, a good check is to work backwards from your answer to see if you end up where the question began.
And ultimately ask yourself whether your answer actually makes any logical sense: If you're claiming that an object is travelling faster than the speed of light then you may well have gone wrong somewhere.