The chip that went grey and said nothing
The worst front-of-house bugs do not throw an error. A low-stock marker lost its colour and every screen still looked completely normal. What catches that is not a test suite.
While making the register's stylesheet smaller we broke something, and the way we broke it is worth more than the fix.
Till tiles carry a small chip when an item is running low — amber for low, red for out. The code that draws it builds the class name as it runs: a stem, then the level. The word "out" happened to be written out in full elsewhere in the source. The word "low" was not.
So the first cut of our tidy-up kept the red one and dropped the amber one. The chip still rendered. It was just grey.
Nothing failed
No error. No blank screen. No slow page. The till opened, the tiles drew, the sale went through, the money was right. A member of staff would have seen a tile that looked entirely normal and would have sold the last three portions of something without the warning that was supposed to be there.
That is the category of failure that front-of-house software should be judged on, and it is the category that almost nothing catches. A test suite checks the answers it was told to check. Ours was green. An error log records things that went wrong loudly enough to be recorded. Ours was empty.
What actually caught it
We drove the real register in a real browser, snapshotted every element's computed style with the new stylesheet, swapped back to the old one on the same page, and compared. Not "does the CSS look right" — is this element on this screen the same colour it was yesterday, 57 properties at a time.
The first run came back with 225 differences. Most were one genuine mistake repeated, which is the usual shape. The grey chip was in there.
The operator version of this
When a supplier tells you a change is safe because the tests passed, the useful follow-up is: did anyone open the screen? Not a staging copy of the screen, not a mock-up of the screen — the screen, in a browser, doing what a member of staff does at 7pm.
Colour, position, the badge that should be amber: none of these things raise an error when they go wrong. They just quietly stop telling your staff something you are paying for them to be told.