Over 25 gb of Space Disappeared
The current version of Time Machine keeps two types of backups.  There are backups that are stored on your computer as well as network backups that are stored on your networked storage (time-capsule, NAS, etc).  You can see these by entering Time Machine and looking at the timeline ticks to the right.  The pink/purple ticks are network copies and the white ticks are the ones on your local computer.  The local backups occur when your network storage is not available, also they may occur more often than the 1hr snapshots set for network backups.  As your hard drive fills up, OS X will start to cut back the amount of space it uses for local backups.  If you want to disable this feature and recover your local disk space, then open a terminal (Applications -> Utilities -> Terminal) and enter the following command followed by your password.

sudo tmutil disablelocal


This should turn off the feature and free up the space.  You can turn it back on with

sudo tmutil enablelocal