Ok - my fault - I guess I can be a spaz
But on at least 3 occasions, I have accidentally hit the "use gems" button in the barracks queue and have unwittingly used gems to create troops.
Sure wish I could have those gems back but there was no recourse. I clicked and it was a done deal.
Sure would have been nice to have saved myself the grief with a confirmation popup. I hope someday you can build this in.
The other issue here is that unless you hover over the button, you don't know how many gems you are spending to do the deed. You only know you have spent alot after the fact.