It was on accident.
My char is lvl 25, and I left my white sign by the Depths bonfire while I farmed rats. Everything was running smoothly (not to mention the fact that a lot of people were active tonight! On PC! Weee!), when out of the blue "Invading another world as a Dark Spirit" started flashing on my screen. I should have just quit the game, but before I knew it I was rising from my white sign as a red phantom!
There was a guy at the other end of the bonfire room wielding a Black Knight Greatsword. He almost immediately started attacking me. I... assumed he was a hacker, preying on n00bs who just wanted a bit of co-op. So I slaughtered him rather viciously with my scythe.
But there were a couple things wrong:
1) He had decent HP, but he went down very easily. You'd imagine a hacker would just insta kill me or have infinite health.
2) If he was really trying to beat-up on newbies, it would seem kind of dumb to choose the one with the Great Scythe. Granted, my armor was start-gear.... but. Great Scythe.
I sent him a "WTF was that?" message after the fight. (Still thinking something smelled fishy.) After chatting with him for a bit, I found out he was just as confused as I was.
Apparently, the guy had been fog-walled like he was about to be invaded. But no invader appeared, and he was annoyed that he couldn't use the bonfire. In desperation, he touched my summon sign to see if it'd help (somehow). It helped. Because after I killed him everything reset. :|
I apologized for killing him. (I would have just left if I knew what was happening..... or if he like..... didn't try to attack me. I'll just chalk it up to shock.) Thankfully he was very cool about the whole thing, and we had ourselves a chuckle.
And I got me some extra humanity and souls. Eheh. whyyyyyy?! I'm a murderer! Nuuuuu!
Guys - does this happen? This occurred on the PC version, but has it happened on other systems?
Very strange night, this was.