Angular momentum needs a no-external-torque axis
The mistake
Conserving angular momentum about an axis with a nonzero external torque.
Why it feels right
Angular momentum sounds like the rotational version of momentum, so it gets treated as always conserved.
How to catch it
You conserve L without checking torque about the exact axis you chose.
The first principle
ΔL about an axis equals external angular impulse about that axis. Pick an axis where external torque is zero or negligible.
worked mini-example
A falling rod pivots about a fixed end. Is angular momentum about the pivot conserved?
Common wrong move
Yes, because it rotates.
Correct
No if gravity exerts torque about the pivot; mechanical energy may be a better tool.
Test the repair ROT-05 · transfer
Make a fresh prediction before referring to the worked example.