I just did something really stupid. After patch 7.2 I've been farming Legionfall War Supplies as much as I could (I don't play that much but love collecting achis).
Out of 3 buildings I liked Nether Distruptor most so I did 5 contributions to it first. This got me an achievement which surprised me. And then I figured there are also achievements for other 2 buildings as well. At that point Mage Tower was already at 85%. I got to 3/5 with Mage Tower before it was built. I figured, no big deal, I'll just make 2 more contributions after it's done. 200 supplies will be wasted, hardly a drop in the sea. But no the game doesn't allow it, and now Magical Contribution achievement is lost to me forever!
Damn it if I was only smarter and gone with the flow.