While converting a 1.6 working copy with build TortoiseSVN-1.6.99.21964-dev-x64-svn-1.7.0-dev.msi
In file 'C:\Users\kueng\nightlybuilds\latest\TortoiseSVN\ext\subversion\subversion\libsvn_wc\entries.c' line 1935: assertion failed (svn_checksum_match(entry_md5_checksum, found_md5_checksum)) The working copy had some subdirectories switched to a different branch, my guess is that this was the cause of the problem. Deleting those directories allowed the conversion to succeed.
