First off, are you using the latest firmware release? It fixes a lot of corruption problems. Secondly, if you are, you are most likely removing the card while it is in the middle of being written to or removing the USB cable in the middle of transfering from your computer.

Remember, even if your OS says that the files are done being written, it's NOT actually the case! The write is finished being flushed to the card reader, not to the card. Leave another 30 seconds or so before removing the card/USB cable.