Lightroom 3 Not Importing All Shots

Yesterday, I was importing files to Lightroom from my Compact Flash card (shot on 5DII) and luckily I noticed that not everything on the CF was imported. About 20 shots were missing. I had already started making selections and luckily hadn't erased the flash card yet! Anyway, I'm just trying to figure out why that would happen. I did nothing unusual from the way I normally import images. I use a USB2 card reader, and all the shots were in the same folder on the camera. When I put the card in the second time, it immediately recognized those shots that didn't import the first time.

Is this a bug with Lightroom or my camera perhaps? Either way it's a little scary as I just happened to catch it this time after importing some 300 shots.