Work only in this folder, with made-up data: no network, no real accounts. Build a small piece of work for a webshop: shipping is
free from 100 euros, otherwise 6.95. Write two sets of tests: a weak one with only an ordinary small and large order, and a
strong one that also tries just below the limit, exactly on it, and zero. Put the expected answers in as plain numbers.

Limits: never change a test to make it green; keep the original work untouched.

Run both sets on the good work and on a copy with one deliberate error, and show me the output, saying which set noticed it.
