svn: The POST request returned Invalid XML - It's always the disk

I was debugging a Subversion 1.7/Apache 2.2 problem where a user could check out code without issue, but any time they tried to update, they got the following error:

"svn: E175002: The POST request returned invalid XML in the response: XML parse error at line 3: not well-formed (invalid token)  (/path/to/svn_repository/!svn/me)".

There wasn't much documentation on the Web, and the documentation said that usually an error would appear in the Apache log indicating a file that the user didn't have permission to access.  I checked the permissions and the server disk space and everything was fine.  I even fired up Wireshark and only saw that the XML that came back contained a null element, which I think more recent versions of SVN has fixed.

I racked my brain trying to figure out the issue, but ultimately determined that the issue was with that particular user's disk quota.  They couldn't create any more files, so they got the red herring error message above.  So remember, for this particular error, it's always, always the disk, whether it's permissions, disk space, or quota.