After winning a battle, I went to make some purchases in the shop. I wanted to buy one of the silver card packs, one of the heralds, and then I wanted to save the remainder of my essence. I had 3411 essence.
I went to the shop, purchased the card pack first, and was then told I had no more essence available :I I then checked my inventory and found the game had granted me three card packs and taken 3000 essence in return- despite the fact that I had only bought one.
I'm not really that mad since I got some nice cards, but this would probably be really upsetting or frustrating to someone with a more limited amount of Essence.