I was working on eliminating a Perforce error being generated in my build script. As I checked in the modified build script into perforce, I added the p4 error message I was trying to fix in the change list description. That way, I could search for it if I saw it agian.
Guess what: When Jenkins tried to build after the checkin, the Perforce plugin for Jenkins must have seen the error message inside the change list and declared the communication with Perforce to have failed and thereofre failed the build.
Luckily, it was an easy fix. I just edited the submitted changelist to remove the error message, and now all iw well.