Please correct me if I'm wrong, but I recently found myself in a pretty confusing situation and now I am unsure how to continue.
I found the jQuery.xmleditor plugin from the UNC libraries and thought it would be useful for one of our commercial products. That's where I typically first check how a library is licensed (in this case it was Apache License 2.0, which allows redistribution under certain preconditions). I started playing around with the plugin and realized that the actual text-based editor (the part I am interested in most) was a derivative of Cloud9, which is licensed under GPLv3. That made me suspicious, because typially GPL requires GPL on derivative projects. So I started a quick web search and found out that there is indeed an incompatibility.
This licensing incompatibility applies only when some Apache project software becomes a derivative work of some GPLv3 software, because then the Apache software would have to be distributed under GPLv3. This would be incompatible with ASF's requirement that all Apache software must be distributed under the Apache License 2.0.
For my current understanding the jQuery.xmleditor plugin is licensed wrong. However, I am not quite sure whether my current understanding is right. Can anybody perhaps enlighten me?