The normal force usually does no work
The mistake
Counting the normal force in the work–energy budget.
Why it feels right
It's a big force, so it seems like it should transfer energy.
How to catch it
You included W_normal in an energy equation for sliding on a surface.
The first principle
Work is Fd cos θ. The normal force is perpendicular to the sliding displacement, so it does zero work; it redirects the motion without transferring energy.
worked mini-example
Work done by the normal force as a block slides down an incline?
Common wrong move
Some positive amount related to N.
Correct
Zero — N ⟂ displacement.
Test the repair EN-02 · transfer
Make a fresh prediction before referring to the worked example.