Mostly everything that annoys me online annoys me simply because it happens in real life, too. I'll just post two instances of this, and I'll let you assume the rest.

First of all, I really hate it when I'm being sarcastic to someone, and they intentionally add realism to it, even though they know I'm being sarcastic. For example:

[Enter Myself]

Me: Would you believe it? AT&T cancelled my phone plan for no reason at all.
Jim: That's such a bummer. What are you going to do?
Me: Well, you see, I've got these Columbians, and five of them, including me, are going to take the store by storm. We'll slit everyone's throats, defile the dead women, and when it's all said and done, I'll go to the clerk that cancelled my plan and I'll shove my phone down her throat, assuming she has one.
Jim: See you in jail.

Now that's completely exaggerated, as I would never bring in sarcasm so early in a conversation and so abruptly unless Jim was being stupid. But anyways, the other thing which I find notable that some people do that irritates me are when people take sarcasm and not only take it seriously, full knowing that I was being sarcastic, but take their idiocracy to the next level by trying to prove their hard-assedness or otherwise in their response. Once again, a scenario.

[Enter Jim]

Jim: Great Jimeny! What happened to my room?
Me: Don't look so surprised, I just moved the bed to the opposite side of the room.
Jim: That's terrible! Why would you do such a thing?
Me: I was just wondering if you'd notice.
Jim: Well, in that case, I'm not really mad at you.
Me: Good, because before you came in I was about to burn all of your salvia, break your records, and use your knife collection to skin your dog.
Jim: If you had of done that, I would have cut your fingers off.

The relation between these two scenarios is that in both cases the person took the sarcasm the wrong way, and thought maybe they could earn something from it (whether it be respect for their morality or fear).

There's a funny story (a really funny story I might add) where someone thought I was being sarcastic about his band and almost broke a bottle of whiskey over my head because of it, but I'll save that for another time.