Catalan spring onions, or "calcots", are a very popular kind of vegetables obtained by special agronomic practices that avoid photosynthesis and subsequent metabolism by the Allium cepa plants. They have been considered for more than a century harmless for pets, as pleasant odour and sweet taste differ so largely from any other onion that can be ingested by children and adults in huge quantities, up to ~3% body weight, without problems being observed. However, the organosulphur chemicals responsible for oxidative damage to canine red blood cells found in this plant species seem to remain present, as two dogs were confirmed to be poisoned after eating left-over "calcots". Both patients presented haemolytic anaemia with Heinz body formation, and eccentrocytosis. Veterinarians and owners should be aware of the dangers associated with this particular onion variety and type, as it is highly palatable to pets and no effective treatment is currently available.