1. Created a project in Jenkins for my trunk code and selected "Accept Integration from Subversion feature branches" in the configuration screen.
2. Created a feature branch from within Jenkins which created a new Jenkins project
3. Configured branch project to "Integrate to upstream upon successful build"
4. Jenkins successfully built the project
5. Jenkins then integrated the changes (there were none) to the trunk and built the trunk successfully
6. Jenkins checked in the latest merged trunk code
7. Jenkins then automatically rebased the branch, built it, and checked in the merged changes.
8. Steps 5-7 repeated over a dozen times before I noticed and turned off automatic upstream integration

I had both projects' build schedule set to Poll SCM every minute (* * * * *). The checkins from the merges triggered each build and kept the loop going.