This is a very weird bug that does involve some amount of both programmer and end user error. The smallest test case I can come up with involves force updating two input files, one of which does not exist, and one of which has been aborted because the FILE does not exist.
Haven't been able to simplify it further. I do see that something may have changed in bro recently.. at least I see different behavior on linux with master and a slightly older version on OS X:
On linux with version 2.4-907 I see:
Where on my OS X laptop with version 2.4-715 I get
And now that I look at recent changes, this looks like it was introduced in