Process:
My last build was for branch1.
I start a build for branch2.
I cancel my build during the svn switch to the new branch on the Windows 7 slave leaving me with a workspace of partial branch1 and partial branch2.
I start my branch2 build again.
Jenkins svn plugin thinks I have branch2 in my workspace so it does an update for the current timestamp. I still have a mix of branch1 and branch2, just updated.
My build is a mix of two different branches.

At first, it's not easy to see what's going on. It wasn't until I went into two different directory trees within that workspace that I did "svn info" and found that I was in two different branches.