What would make songs dissapear?

Just gone.
If this isn't the right place to post this, please move it. Thanks.

I copy most all my CD's to the computer and do the bulk of my listening there.
Went to make a CD of stuff to listen to in the car the other day and found 4-5 songs from different CD's were just gone. Searched the hard drive and they don't show up at all. It's ok if I still have the original CD, but that ain't always the case.
Seems weird that random songs would just -POOF! Be gone.

What would make this happen?
Windows7 on an HP desktop.