What has (most likely) happened is that the .svn/entries file has gotten corrupt somehow; this can happen if a svn client dies halfway in a commit, or for other reasons, but that is not what this post is about.

Recently got the error message “svn: Checksum mismatch for ‘/var/www/magento/app/locale/sv_SE/template/email/sales/.svn/text-base/testfile.svn-base’; expected: ‘2b991059e896f837f33875df8070e1ab’, actual: ’71d7d03cf41d65b6c28471dd2e96c2e1′”

A quick fix this issue is to

• Remove the .svn directory from the sales folder.
• Move the folder sales to sales_real
• Update the email folder (this will recreate the sales folder)
• Move all the files from sales_real to sales
• Commit sales
• Delete sales_real

Obligatory disclaimer: This worked for me, it keeps the file history and all should be well.
If the sales folder would have been larger (and or my connection to the svn server slower) perhaps some other solution is preferred; but this was quick and worked.

Credit to dr. chuck for the original post