The mark failed
On this page9
Never applied
The most common failure, and it happens on the first run when the line is busy and the step is new. The units are compliant in every other respect and the one variable mark is missing.
Smeared or illegible
Ink on a glossy curved surface, applied at speed, that does not key. The requirement is to clearly display, which is a judgment about the finished unit rather than about whether a mark was made.
Rubbed off in transit
Uncured ink against a case wall for three days. It passes inspection at the line and fails at the shelf, which is the worst kind of failure to detect.
Printed into the artwork
Which makes every unit carry the same date and defeats the entire purpose. It is an easy mistake for a designer who has not been told the mark has to vary.
A code nobody can resolve
The scheme lived with the person who set it up and they have gone. The mark is present, compliant and useless.
Fix it with a line check and a document
A check that reads a unit from each run, and a one-page document holding the scheme. Both cost minutes and both are the difference between a code that works and a code that is decoration.
And keep a marked retained unit
From every batch, filed with the record. It proves what was applied and it is the reference when a question arrives two years later.
It is also the failure nobody reports
A customer will tell you a sprayer stopped working and will never mention a missing date code. The only way it surfaces is a line check or an inspection, which is why the check has to exist.