There is a few catfish out there that are pretty common and are VERY nice in color of patern that will get HUGE. but are sold small and cheep so people not knowing what they are getting will buy them soon to find out that they can not house them b/c they wont stop growing.

A few to watch out for and a very basic size they can get....

Red tailed cat. 5'
Tiger shovel nose 4'+
RTCxTSN 4'+

These are the big guys and I will list more when I get the chance. But more LFS sell these guys very small and for very little money. I have personally seen RTCs at 3" sell for $11 and trust me they are cut when they are small but will soon become a hadfull when they grow.

Another common catfish to watch out for is the Irridescent Shark, which can grow to over 3' long.
Great post Koop.

koop ... i hear that those Ripsaw (aka niger, or Turushuki catfish) can get pretty large

also, giraffe cats, paroon "sharks", marble catfish, channel catfish, common pleco's ....
just off the top of my head

anyhow, i actually ENCOURAGE everyone to buy these fish! (then when they get to big for your tanks, sell them to me and Koop )