Wait, What?

[Xaiux]

So I was doing a mission, I think it was on Eris. Anyway, what happened was that I was playing Volt, and spamming Shock at a bunch of Grineer. The mission was somewhat laggy, and when I ran out of energy, the bar displayed -5 energy. Another player had Energy Siphon on, and so it regened back just fine, but that was the first time I'd ever seen that in about 200 hours of play.

[Dtexas]

Yes, you can make energy go into the negatives with very spam-able abilities. Ive gone -100 before using Saryn's molt.

[Eliard]

It's just a result of your side of the game using energy faster than the host can update it - by the time the host tells you that you're out of energy, you've already used more energy than you had when you started spamming (e.g. using 125 energy worth of Shock when you only had 100 energy). This can happen anywhere if there's enough lag, and though it's obviously not intended behavior, it's not exactly a bug either. Don't worry about it. :P