I set up a conditional build step to run a 'Clean' target once a week on our nightly build. The night following the successful clean failed with a NullPointerException:

Exception caught evaluating condition: [java.lang.NullPointerException: null], action = [Fail the build]
Build step 'Conditional step (single)' changed build result to FAILURE
Build step 'Conditional step (single)' marked build as failure
[locks-and-latches] Releasing all the locks
[locks-and-latches] All the locks released
Archiving artifacts

I have not been able to initiate a successful build since this first occurred. The same error prevents all subsequent tasks from initiating.