The most common one is "A little boy named Craig Shergold is dying of cancer. His last wish is to make it into the Guinness Book of World Records for "most postcards" before he dies. Won't you help by sending a postcard to <address>?"
This is true. Or was true. The problem is that Craig got better. And he made it into the GBWR. And he's still getting postcards. Lots and *lots* of postcards. Literally tons of them. Don't send him any. Thanks.
This is related to the story of "My sister's in the hospital and I can't pay her bills anymore. Will you help by sending money to <address>?"
Sometimes this is true, sometimes this isn't. I can't tell you what to do here. I don't send them any money .. I'd rather donate to places where I know better what the money's buying. But feel free to do so. Just please don't post these things to inappropriate places. People who do so are often viewed as rude, and often thought to be scamming. Which they probably are.
Next patient, please.