There's a story that I've heard - heard it from a few different people, each saying it happened to them, which makes it more like a phenomenon, I guess. Several folks over the years - co-workers, casual acquaintances, zoo visitors - have told me stories about the pet turtle they had as a kid. They all took their turtle outside to play on a nice day (good for them, getting some Vitamin D for the shells) and set it in the grass. Then they turned their back ("But just for a second!").
When they turned around, ol' Shelly (either that or one of the ninja turtle names) was gone. It just never occurred to them that their pet turtle might, you know, move.
One of the funniest sights of zookeeping is watching a hungry, hormonal, or otherwise motivated turtle or tortoise haul shell. Sure, they won't win any races at the zoo, and they can't maintain it for very long, but they can really surprise you when they get moving. Couple that with the fact that you probably weren't expecting to have to chase a tortoise, and you can easily see how someone's pet turtle might run away if left unattended. Especially if it's a warm summer day and there are lots of yummy things to eat.
Still, as many times as I've heard this story, or some variation of it, I still hear endless jokes about how slow turtles are, with many people seeming to believe it. Sloths are like that too. I was cleaning our sloth exhibit once and heard a mother tell her children that it would take about one year for our sloths to go from one end of their (comfortably sized) exhibit to the other. I've seen them do it in under a minute. When they are motivated, of course. Which they seldom are...
Visitors love to embellish about our animals. Sometimes, that can lead to disappointments, with animals unable to match the stories. No, chameleons can't instantly change color to match anything - if you put them on a chess board, they won't immediately turn into black and red squares. No, cheetahs can't run 150 miles an hour, they top out at 70. No, black mamba venom won't kill you in two seconds. I met a guy in college - my roommate, actually - who was disappointed to learn that giraffes were "only" 16 feet tall... he thought they were 50 or so.
There's no reason to exaggerate the strengths (or deficits, I've heard some remarkable beliefs concerning ostrich stupidity) of animals. The truth is amazing enough. Especially when it defies our beliefs.