The fill was wrong
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A size that is not authorized
The worst outcome and the most preventable. Twenty-five numbers below three litres and even litres above four, and anything else may not be sold.
The gap between 3 and 4 litres
Catches people who reason that if 3 is allowed and 5 is allowed then 3.5 or 4.5 must be too. The list ends at 3 and the rule starts at 4 in whole litres, and nothing lives between them.
Nominal capacity is not fill capacity
A bottle sold as 750 mL may brim at 790 and look wrong filled to 750, or may struggle to hold it with headspace. The standard is about the contents, not the bottle's name.
Headspace shows on clear glass
Two wines at the same declared volume can look like different quantities on a shelf, and the customer believes their eyes.
Temperature at filling
Wine expands and contracts over a working range, so a line filling accurately in the morning can drift by the afternoon, and the drift is usually toward a short measure.
Check before the mould
If a custom bottle is involved, the standard of fill and the actual fill level are settled at the mould stage and are extremely expensive to change afterwards.
And verify by weight
With the density of the actual wine rather than water. Alcohol content changes density, so a checkweigher set for one product will be wrong for the next.
Even litres means even, not approximately
A 5 litre bag filled to 4.8 is not a 5 litre container filled in even litres. The rule is about what is actually in it, which makes fill accuracy a compliance question at large formats rather than a quality one.
Keep a filled retained bottle per run
Unopened, with its label and its fill level visible. It is the only evidence of what actually shipped, and the question always arrives long after the run.