Incorrect free space indication in Finder

Apparently there is an unpleasant bug in Finder with Lion and Mountain Lion. Sometimes Finder status bar shows you much more free space than you actually have. 

To be sure, compare Disk Utility indication and Finder indication. Disk Utility is always correct. 

If you see that the numbers do not match, don't panic. It is actually fixable. There can be two reasons for Finder lying.

1. HD is corrupt. Try to repare it with Disk Utility, you may require reboot in Recovery mode to do that. To do that, press CMD + R when rebooting. Choose Disk Utility from the Recovery menu and go from there. Sometimes (happened to me) one iteration is not enough. It took me tree attempts to fix my SSD corruption. Sometimes you cannot recover the drive. I hope you have your Time Machine backups to restore from them. If not, boot, try to backup all you can and re-install.

2. If your drive is OK, but Finder still shows you ridiculous amount of free space, that might be Time Machine to blame.

In my case, when Time Machine automatic backups are enabled, Finder begins to lie. It seems that it add backup sizes to the free space, even if Time Machine drive is an external one. To fix this annoying issue, unmount Time machine drive, turn automate backups off and re-launch Finder. To restart Finder, do ALT + two finger click (right click with a mouse) and choose Relaunch.