Free disk space that isn't there!

[Henry C. Schmitt]

I recently ran into a problem and wonder if anyone knows a solution.
While working on compiling a couple large programs, I started getting
the error: Out of disk space on file system /usr (or something to
that effect).  I did a df which showed that /usr had over 10,000 free
blocks (> 5 MB!!).  However, doing a df as a normal user (I was root
for compiling) in another window showed 0 free blocks on /usr!!!

The other file systems showed discrepancies of 15-30% in free space
depending on whether or not I was root.  Why is there free space only
discernable by root, but not accessable by root?  What's going on
here?  I'm confused.
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