Had this happen to me twice when I was queue-ing Durance's Barbs of Condemnation.
It was caused when I queue'd several actions, including Barbs of Condemnation and then for whatever reason he continued to cast after combat ended. It wasn't a big deal for the AoE buffs since those took place as usual, but it's odd that Barbs was still cast even though the target was no longer there.
[unrelated side note: it sure would be great if I could see what was queue'd.]