French border and customs authorities check pet travel documents at entry points, and a pet that doesn't meet the requirements — a missing microchip match, a vaccination given too recently, an invalid or missing passport or health certificate — is treated as non-compliant. According to French guidance, the realistic consequences range from the pet being refused entry, to being returned to its country of origin, to being placed in quarantine, all at the owner's own expense; in the most serious cases involving genuine disease risk, euthanasia is on the table.
A separate, equally serious category of non-compliance is bringing in a dog that falls under France's category 1 restrictions. Since import of category 1 dogs is prohibited outright, discovering this at the border isn't a matter of paying a fine and continuing — the dog cannot legally enter the country, and the practical outcome is the same range of return, quarantine or worse described above, layered on top of the separate legal consequences for attempting the prohibited import.
None of this is designed to catch out an owner who made a genuine, minor timing mistake — a rabies vaccination given 19 days instead of 21 before travel, for instance, is a real problem worth sorting out with a vet in advance rather than discovering at the border. If something has gone wrong before you travel, contacting a vet or the relevant French consulate ahead of time is a far better position than hoping the check won't notice.