Invisible opponents, fault from my side, hacks or neither?

I just started playing survival games, and occasionally, in sudden death, there would be an invisible guy shooting arrows.

Now, I'm not sure whether that's a problem on my side, bad rendering, glitching, texturepack fault, or is a person using hacks? Or is it something else?

Thanks for replies!

EDIT: Really unsure why did my thread got moved to bug reports. It's not that ALL opponents are invisible, just occasionally one happen to be. If this in fact is a bug, then I give you my apology for making a thread about already known issue.

 

Well, this happens on really random occasions. Not sure what the problem is, because this also happens to me as well, and I have a fairly decent computer and internet. I think that this is just a Minecraft glitch because I see this happen on other servers.