An timed exam has predictable methods for targeting a 40-60% error rate:
1. Potential subjects too broad to fully study.
2. Problem has an uncommon reference.
3. Misleading anchoring.
4. Confusing or complex wording.
5. Irregular problem difficulty (easy...or missing the trick?).
6. Shortcut required or problem too long.
7. False answer match
8. Unexpected numerical precision.
9. Inexact correct answer.
Most of these are self-explanatory, but #3 is a bit cryptic. "Anchoring" is a cognitive bias toward a particular solution path (the incorrect one) resulting in mentally excluding all others (including the right one) and so freezing up.
Regarding #1 and #2 it's very easy to waste time on a problem you don't really understand rather than cutting one's losses and spending the extra time checking problems one does know how to do. Ironically, limiting the open-book option with the single allowed reference on the CBT will help avoid this problem.
