The most common mistake, in my view, is treating chemistry as a subject that depends almost entirely on memorisation. Many students spend a great deal of time memorising organic reactions: which reagent gives which product, or which conditions lead to which transformation. This approach can sometimes work well in A-level or IB examinations, because those exams often test familiar reaction types relatively directly. However, once you move on to C3L6 or UKChO, the question may present a completely unfamiliar molecule. If your first reaction is still, “Have I memorised this reaction before?”, it is very easy to lose direction immediately.
The second common mistake is treating the number of questions completed as a measure of progress. Many students preparing for UKChO start with the 2003 paper and work through every year until the most recent one, eventually feeling a strong sense of achievement because they have completed more than twenty past papers. However, UKChO and C3L6 questions change every year, seldom repeat. So, the important question is always how many reusable ways of thinking you have extracted from them.
The third common problem appears in physical chemistry and calculation-based questions. Many students feel that they fully understand a solution when reading the mark scheme, but when they attempt the calculation independently from the beginning, they make frequent mistakes. What often separates stronger candidates from weaker ones is whether they can maintain accuracy throughout a long sequence of calculations and deductions. Mixing up kJ and J, forgetting to convert cm³ into dm³, confusing mol with mmol, rounding intermediate values too early, or dropping an exponent during algebra may each seem like a “small mistake”. But in a long calculation chain, one small error can affect every subsequent part of the question.