When trading with a villager for an item with a discounted price the trade will not go through if you have less emeralds than the original price.  For example if the original price is 15, and the discounted price is 10 but you only have 10-14 emeralds, the trade will appear as though it works. However, when you click the item to put into your inventory, it pops back into the trade menu along with your emeralds.  Unlike other bugs I've seen the price does appear to be correct.  If you have 15 emeralds in the case above, it will only take 10 and the trade will go through