gistfile1.txt

<?php

// Imagine you have a class that memoizes some data statically (let's assume
// the data is computationally expensive to load)

class ExpensiveDataHelper
{
    protected static $expensive_data = null;

    public function getExpensiveData()
    {
        return static::$expensive_data ??= $this->loadExpensiveData();
    }

    protected function loadExpensiveData()
    {
        // ...
    }
}

gistfile2.txt

<?php

// Somewhere in a test, you create data that the helper will load and
// then call some code that uses the helper

public function test_something_that_uses_expensive_data()
{
    Data::factory()->someState()->create();
    $data = ExpensiveDataHelper::getExpensiveData();

    // Perform assertions on data
}

gistfile3.txt

<?php

// Now in another test, you create *different* test data and run your tests.
// If the data has already been memoized, your test will fail because
// it's not truly isolated from your other tests

public function test_something_that_uses_expensive_data()
{
    Data::factory()->someOtherState()->create();
    $data = ExpensiveDataHelper::getExpensiveData();

    // ...
}