The Pentax K-x, a DSLR that Pentax stopped making almost two years ago, has a problem. Some have experienced this problem right out of the box. For others, it doesn't develop this problem for some time (if at all). It first started happening to me recently on my 2 1/2 year old and well-used K-x.
The camera will say "Memory Card Full" when in fact there is plenty of room on the card. You may also see a message that reads "Memory Card Error." When this happens the camera will not record the image onto the card.
Removing the card and reinserting, removing and reinserting the batteries, even reformatting the card doesn't help.
I first had this problem one month ago on a card as old and as used as the camera itself. I figured it must be the memory card and so I put a brand new one in.
One month and about 200 photographs later, the same problem occurs (and at a bad time, too). So it must not be the card.
A quick Google search reveals that this is a common problem with this camera. I'm actually pretty lucky that my camera waited over two years to start acting up.
Solutions didn't come up right away. Some people stopped experiencing the problems after switching to higher-end "Class 10" memory cards. Others gave up on their cameras altogether.
One easy fix I did find is to update the firmware. Version 1.03 apparently fixes the problem (click the link and follow the directions). I did this and reformatted the card. So far, so good! However, only time will tell if this really fixed the problem or not.