When I check CA answer sheets, the most common feeling I have is not "this student doesn't know the topic". It is "this student knew it, and still lost marks". After checking a lot of Foundation, Inter and Final papers, the same few mistakes come up again and again. None of them needs more studying to fix. They need practice in writing the answer the way it will be marked.
Here are the ones I see most often, paper by paper.

Accounts and Financial Reporting
Missing working notes. A student gets the final figure right, but the calculation behind it isn't on the page. If the figure is wrong by even a small amount, there is nothing to give step marks for. ICAI's own suggested answers almost always show working notes, and that is the format to copy.
Wrong or incomplete format. A Cash Flow Statement without the three headings, a Balance Sheet that ignores the Schedule III order, or a journal entry without a narration. Each of these costs a little, and the little adds up across a paper.
Ind AS answers that jump to the number. In FR, the question usually wants the standard, the reasoning and then the treatment. Students who write only the accounting entry lose the marks for the explanation.
Law and Audit
Conclusion without the provision. "The company can do this" earns very little on its own. The structure that works is simple: the relevant provision, the facts applied to it, then the conclusion. Three short paragraphs beat one long one.
Copying the question back. Many answers spend the first five lines restating the facts. That time is better used on the provision and the analysis.
Vague Audit answers. Writing "the auditor should be careful" instead of naming the actual Standard on Auditing and what it requires. If you know the SA, write its number and the requirement.
Taxation
Skipping the steps. Tax computations are marked step by step. A student who writes only the final tax liability, even if correct, is taking a risk; a student who shows each head of income and each deduction can collect marks even with an arithmetic slip.
Not stating assumptions. Where the question is open to two readings, a one-line assumption ("assumed that the assessee has opted out of the new regime") protects you.
Across all papers
Running out of time. This is the biggest one. Students who have only practised individual questions often find the last question untouched in a full paper. A three-hour attempt at home is the only way to find out your real speed.
Leaving no space for corrections. Cramped answers are hard to read, and hard-to-read answers are rarely marked generously.
Not numbering answers clearly. Make it easy for the person checking to find each part.
A simple routine that fixes most of this
- Write at least one full paper per subject under exam conditions before the exam month.
- Compare your answer with ICAI's suggested answer line by line, looking at structure, not just the final figure.
- Ask someone else to mark a couple of your answers. When we check our own work we read what we meant to write, not what is actually on the page.
- Keep an error sheet: one line per mistake, and read it before the next paper. This helps to improve presentation and figure out mistakes before the real exams.