I already submitted a bug report here, but wanted to post here to make sure I wasn’t missing anything.
The file in the bug report is a very simple setup that contains an armature that plays actions on layer 0 and layer 1. Normally actions on higher layers should play on top of actions on lower layers, but on newer versions of Blender (2.76 and higher) actions on higher layers don’t seem to play at all if there is an action playing on a lower layer.
Was there some interface or functionality change to how action layers work that I missed, or is this indeed a bug? It seems like a fairly significant bug to have been overlooked this long. I only just now noticed it as I’ve still been using 2.75.