Banked-curve friction direction depends on speed
The mistake
Assuming friction always points up or down a banked curve regardless of speed.
Why it feels right
Friction gets memorized as 'opposes motion,' but the car is moving around the curve, not up or down the slope.
How to catch it
You choose friction direction before comparing speed to the no-friction design speed.
The first principle
On a banked curve, friction opposes impending slipping along the surface. Below design speed the car tends to slide down the bank, so friction points up; above design speed it points down.
A car takes a banked curve slower than the no-friction speed. Which way does static friction point?
Common wrong move
Down the bank because the car moves forward.
Correct
Up the bank, preventing the car from sliding inward/downward.
Make a fresh prediction before referring to the worked example.