A lot of people don't realize how many fish in the aquarium trade get REALLY big if they stay alive long enough, just a few fish that get over 12" long: single tail goldfish and koi, common and leopard sailfin (petco sells as 'Columbia') pleco, knifefish, clown loach, tinfoil barb, bala and rainbow shark, giant gourami, red tailed catfish, pacu
It's so true! I love single tails because of their size, but it's amazing how many people just... don't know. And it's rough for everyone involved when things like that do happen. I guess it's really up to the person getting the fish to really look into how much they're signing up for and for the staff, whoever they might work for, to try and enforce that.
I always thought bala sharks were precious, but even I was impressed when I looked up how big they got!