Easily triggered by adding a new file with a tag that matches a rule's inputs. See the "wildcards-and-rules" blackbox autotest for a demonstration.
A quick first debugging session showed that Executor::executeRuleNode() leads to new connections being created to artifacts that never show up in the Leaves data structure. As a result, the output artifact's transformer is never run, the artifact's build state stays at "Buildable" forever and when the Executor finishes, there are unbuilt products left even though no error has occurred up to that point.
I'm not sure whether the rule application itself is buggy or whether change tracking should have done something prior to that point.