Went to Pets at Home to have a nose at the tanks. I found the following 'gem':
Suggestions for fish for a 25 l cube aquarium printed on the box.
Goldfish
Option 1: Single tailed goldfish: max 3x small goldfish. (They will require a larger home as they grow)
Option 2: Fancy goldfish: max 3x small Orandas etc. (They will require a larger home as they grow)
Ok, I suppose they suggest a bigger tank for the future.
Now the real OMG-gems that don't take into account schooling fish / suitability / tank mates:
Temperate
Option 1: 4x white cloud mountain Minnows, 4x variatus platys, 4x danios, 1x Hong Kong plec
Option 2: 4x rosy barbs, 2x paradise fish, 2x variatus platy, 1x sucking loach, 2x shrimp
Option 3: 4x variatus platys, 3x rosy barbs, 4x danios, 1x apple snail
Add a heater to go tropical
Option 1: 1x male fighting fish, 6x harlequins, 3x corydoras catfish, 3x platy
Option 2: 4x male guppy, 4x assorted danio, 4x glowlight tetra, 3 x corydoras catfish, 1x apple snail
Option 3: pair of dwarf gouramis, 6x neon tetra, 3x corydoras catfish, 3x platy
I am just imagining someone looking at those suggestions, buying a tank and chucking their options in. Fail big time