Probably a not so obvious side effect of alt-right-clicking on an item building in a queue to "send it to the back" is that the item loses all it's stored BP.
It isn't mentioned in the tooltip, and it's an obvious effect once you've done it, but given the description I did make the assumption that I simply wouldn't lose all the build up resource.
Thankfully I don't use the alt-key for much or I probably would have found out by accident by accidentally right/left clicking on them with alt held down a while ago.
The bug is either that it shouldn't happen, or that it probably should mention in the tooltip you're going to lose resource. "cancel building and send item to back of the queue" would probably be something like the phrasing (the controls documentation doesn't mention this either).
