Over-learning everything. Many students tend to make this mistake of over-learning everything which lands them nowhere except the state of bewilderment.
Only mugging and not understanding concepts.
Feeling depressed due to low scores in tests. Take this as way to learn from your mistakes.
Taking excessive mock-tests / Not taking mock-tests at all
Ignoring the marking scheme. The preparation in fact must always be according to the marking scheme and the fact if there is negative marking in the exam.