So, sometimes I have a full bank of courage that I want to quickly spend all at once, and I go through my list of enemies and start a couple of fights. I am, however, a slightly clumsy person from time to time and I have (twice now) accidentally spent 20 diamonds to fight an extra time.
I know I should be looking, but I feel like there should be a confirmation popup to protect people from accidentally spending the diamonds they've either spent real-life cash on or have been carefully saving up for a particular purchase. This can be especially frustrating when a decent number of diamonds have been wasted on something inconsequential such as fighting another opponent instead of waiting 9 minutes (and infuriating when you don't even win the fight anyway!)
Just a last line of defence for idiots like me who click before I look at the button properly...