I don't encounter many glitches on Destiny, but yesterday I was patrolling Mars when I noticed an invisible Harpy in the buried city area. You could only see the beam of light when it fired, otherwise there was nothing there. So it was more of a ghost harpy than an invisible harpy because it wasn't actually there. I threw grenades, I even used fist of havoc, but the thing wouldn't die. I recorded a video but sorry about the quality, my phone was built in 354 BC. Hopefully it'll be enough to illustrate what was going on.