My connection was poor at the time, and as i was starting a dungeon i was given the "retry" screen to start the dungeon. 

I kept clicking retry because i didn't want to lose the energy, but i gave up and re-logged in (which worked fine....).

When i logged back in, all my energy was gone and i can only assume each time i clicked retry that it submitted a new request to access the dungeon and spent the energy.