My Subversion repository (1.5.5) is setup in a fairly typical fashion:
I have two working copies: one from the trunk and one from a branch. If I'm in my working copy from the trunk, and I compare a file to the same file from the branch, it works as expected. However, if my working copy is from the branch and I try to compare with the trunk, I get an error stating "Couldn't find repository location for file ... etc.". In that message, both the full path to the file and the URL of the trunk are correct.
Sound like a bug?
OS X 10.5.6
Subversion client 1.5.5