The file in the .svn directory that keeps track of what you have checked out, when, what revision, and from where, has gotten corrupted somehow, for that particular file.
This is no more dangerous or critical than the normal odd file problem, and can be because of various problems, like a subversion program dying mid-change, power-disruption, etc.
Unless it happens more I wouldn't make much out of it.
It can be fixed by doing what you did, make a copy of your work-files, check out a fresh copy, and add the modified files back in.
Note that this might cause problems if you have a busy project where you would normally have to merge in changes.
For instance, you and a collegue both check out a fresh copy, and start working on the same file. At some point, your collegue checks in his modifications. When you attempt to do the same, you get the checksum problem you have. If you now make copies of your changed files, do a fresh checkout, then subversion will lose track of how your changes should be merged back in.
If you didn't get the problem in this case, when you got around to checkin in your modifications, you would need to update your working copy first, and possibly handle a conflict with your file.
However, if you do a fresh checkout, complete with your collegues changes, it now looks like you removed his changes and substituted with your own. No conflicts, and no indications from subversion that something is amiss.
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