Tests that would have caught the last bug

Write the small set of tests that would protect this code from the bugs it has actually had.

The code: <paste the module or functions>
What it's supposed to do: <one paragraph>
Bugs it has had, or nearly had: <describe each in a line>
Testing setup: <framework, where tests live, an example existing test if you have one>

Produce:
1. The 5–8 behaviours most worth protecting, ranked. For each: why it matters, and which past bug it relates to, if any.
2. One test per behaviour, in my framework and style, each with a name that reads as a sentence ("rejects an order with the wrong email").
3. For each test: the setup, the action, the assertion, and nothing else. No shared magic; readable on its own.
4. Edge cases you deliberately chose not to test yet, and why.
5. One test that would currently FAIL if I'm right about a bug that still exists. Say which line of code you think is wrong.

Do not aim for coverage. Aim for the tests I'd be grateful for at 2 a.m.