I rarely see it happening, but in some cases the parent build starts not on the label that I selected on the configuration (master (the master Jenkins node)) but on some other slave node.
I suspect it that it is some kind of fallback or race condition situation, as in most cases, this works just fine, I only spotted this, because our FreeBSD slaves has a bad IO perf, and the SVN checkout there takes more time, than the actual job execution.