Finder Reports Disk is Full, Other Tools Don't Account for it
I had similar issue where the used disk space shown in Finder was about 20 GB more than what the size of all files should have been.
When I tried verifying the main OS X volume (like Macintosh HD; not the drive) in Disk Utility, there was an error about an invalid free block count. I was able to get rid of the error and free up the disk space by starting up from the recovery partition and repairing the volume.