A handful of times I have encountered people with the mistaken belief that CO2 euthanasia is painless because "that's how we kill lab rats" and "that's how the vet put my dog to sleep." They're wrong about CO2 being painless of course, but it seems to be a somewhat common misconception. Also many people get carbon monoxide and carbon dioxide mixed up, adding to the confusion.
More education is necessary. Right now a lot of people have no clue that they're wrong.