This typically happens when newish users think that clicking on Import and being able to "get to work" means the entire import process is complete.
Very often it isn't.
The only way to make absolutely sure your imported clips are completely imported is to open the Render Manager (click on the circular render progress indicator on the Dashboard in the center of the interface) and then make sure all import process progress bars are complete before you close X down. If you do this consistently, the problem will go away.
Thanks for the replies, it makes a lot of sense. The thing is the footage was given to me already in a hard drive. The camera person was the one who imported it from her camera. Now I just learned that she no longer has the original footage in the camera cards. What else can I do to fix this?