Note: owners of Canon PowerShot S400 cameras, please see this first (you just might get this problem fixed for free!).
For all others, Oh No! Your favorite compact flash camera all of a sudden has started giving you CF memory card errors. You've tried several different memory cards, and it's still the same thing. Your camera is inoperative due to this error. Don't fret, you just might be able to fix this problem yourself.
Take a look inside your camera's CF card slot. It's very likely that you may have a bent pin in there. If so, remove batteries and card. Use a small/skinny jeweler's flat screwdriver to bend the pin back as close as possible. The jeweler's screwdrivers that you see at the dollar store will work fine. Another tool that I sometimes use is a stiff wire with a bent hook on the end to get very flattened pins started. Take your time and use as much patience as you can muster (take a breather in between if needed). Just keep nudging it upward/straighter a "tiny bit at a time". Whatever you do, don't try to unbend it with one push/nudge, you won't be able to do it.
It doesn't have to be perfect, just as close as you can get it. When it's the best that you can get it, take your flash card and "SLOWLY" insert it. The card itself should fine-align the pin. Remove the card, and verify that the pin is still straightened. If so, reinsert the card and try it out.