Growing up, I always thought the phrase "Be careful what you wish for," was a strange one, mainly because most examples I saw of wishes gone awry weren't what the wisher wanted in the first place. I completely understand being careful what you wish for, because you may not like it as much as you think. The classic example is King Midas, who doesn't stop to think about what it would really be like to have everything you touch turn to gold. Then there are examples of people who think they want something, but then learn that the reality of their wish isn't the same as what they imagined. But most of the time, when the "be careful what you wish for" lesson showed up in kids' stories and shows, the person granting the wishes didn't give the wisher what they really wanted. The wisher's mistake wasn't that they made a certain wish, but that they weren't specific enough when converting their thoughts into words.
Here is an example of the difference: In the Disney Channel movie, The Other Me, a boy clones himself and gets his new twin to go to school for him and fulfill his responsibilities while he stays home and watches TV. This was a dream come true for him at first, but after a while he got bored of doing nothing and didn't like someone else living his life for him. This is a good be-careful-what-you-wish-for example because he learned about the downsides of what he had wanted.
In another Disney Channel movie, Sixteen Wishes, a girl gets 16 wishes granted on her 16th birthday, one of which is for people to stop treating her like a kid. When the wish is granted, she actually becomes an adult, so that her parents pull her out of high school and expect her to get a job and live on her own. As soon as she realizes what's going on, she tries to take the wish back because it isn't what she wanted. She wanted more freedom to do what she wanted, like staying out later, but not to actually be an adult. While this is portrayed as a be-careful-what-you-wish-for moment, it really isn't, because what she really wanted wasn't what she got. No matter what kind of lesson she learns about appreciating what she has, the real problem was that she didn't get what she actually wished for. Sixteen Candles is the only example I can think of at the moment, but I've seen a lot like this, and my thought was always the same: that you can't say "be careful what you wish for" to someone who didn't get what they wished for in the first place.