I had a similar issue in Yosemite. The Finder was showing 150 TB (yes, terabytes!) of free space. About This Mac had a sane (and presumably correct) value for free space. Disk Utility repair and Disk Warrior directory rebuild didn't help, but turning off time machine and restarting fixed it. Things still look OK after re-enabling time machine, but this definitely looks like a bug to me.
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Also just found these two threads with more background:
https://forums.macrumors.com/threads/1421824/
https://discussions.apple.com/thread/3983143
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Also just found these two threads with more background:
https://forums.macrumors.com/threads/1421824/
https://discussions.apple.com/thread/3983143