This bit me because I'd assumed each job component was run in isolation and variables were initialized with sensible (aka empty) content. In my case it fortunately only affected the reporting of each piece of the job system, but definitely could have affected larger pieces of the system.
The lessons? 1) Always explicitly initialize your variables; you don't know the ultimate context they'll be run in. 2) Individual component testing is no substitute for testing a system as a whole; you can reveal bugs that would otherwise slip through.