Series and parallel don't combine by 'adding up'
The mistake
Adding resistances in parallel (or reciprocals in series).
Why it feels right
Series and parallel formulas are often recalled without first tracing the circuit topology.
How to catch it
Your parallel resistance came out larger than either resistor.
The first principle
Series: R = R₁ + R₂ (adds). Parallel: 1/R = 1/R₁ + 1/R₂ (equivalent is smaller than the smallest branch).
worked mini-example
Two 4 Ω resistors in parallel. Equivalent resistance?
Common wrong move
8 Ω.
Correct
2 Ω, since 1/R = 1/4 + 1/4.
Test the repair CIR-01 · transfer
include Ω
Make a fresh prediction before referring to the worked example.