Here is a glitch I encountered before, but didn't understand originally. Now I can confirm. It has no real impact on gameplay, but it is a good FYI for the programmers

energy caps off at 5 for arena and 10 for epic bosses. if you receive a progression reward (+5 arena or +10 epic boss) and stop playing you will regenerate your energy and get the reward. When you log on a couple hours later you will have 10 arena energy and 20 epic boss energy. This does not give an edge to anyone, but is an interesting FYI.