✘ TOO SMALL - a lot of carriers sold in stores these days are way too small! When shopping for a new carrier for your kitty, make sure they have enough space to stand/turn around comfortably! You may have better luck shopping online where there are more larger carriers to choose from! Avoid triangular shaped 'collapsible' carriers especially where they likely won't be able to stand comfortably!
✘ UNSTABLE/UNSAFE - kitties need to feel secure in their environment, which includes when they are traveling! Being in a backpack like carrier is bouncy, because of how we humans walk, and although us humans might think it's cool for them to see around them, cats actually prefer small, dark spaces where they can feel warm, secure, and hidden from predators. NEVER use a cardboard carrier which can break easily, or your kitty can break out of!
✘ UNFAMILIAR - when it's time to go to the vet, a kitty is always going to feel more comfortable with a carrier they are familiar with, and that smells like them! Smells are SO important to kitties - always keep a familiar item in their carrier like a favorite bed or blanket, or toy! A shared carrier that smells like other kitties (especially unfamiliar ones) and is dirty/dusty because it's kept outside in the cold Michigan garage all year, and is only brought out on 'scary' vet day is NOT going to be a good situation for you or your kitty.