Actually, my worst rp experience was back in the earlier days, way, way, way back in the day when MSN chats were still around and that was my spot. I was still learning, but even I had enough common sense to know that you didn't jump into someone's storyline without either asking, or being invited.
My friend and I were considered the 'elitest' of our little chatroom, because we usually had huge posts that took several minutes to get through because of the character limit that came with the chat room. We'd usually type out our lines until we reached the end and add a little notation at the end of the paragraph to signal that we were continuing on. I can't tell you how many times we'd be interrupted by someone jumping into the middle of our posting to do something stupid! One time our characters were off away from the main scene of the chat room rescuing another character. During one of his posts, someone decided to molest my character, who was not only in a relationship with my friend's character, but she was not anywhere near the guy, and was in fact in the middle of a fight. So, we politely pointed out OOC that his actions were being ignored, and why. Rather than simply forgetting about it and doing something else, the guy magically teleported his character to where my character was, and continued on with his ridiculous molestation.....This continued on for several minutes, leading my poor character to be stuck fighting a possessed guy who was trying to sacrifice a kid to gain more power, but fighting off the guy molesting her as well. Naturally the guy ignored every move that I made, and with my friend in the middle of his own fight, he had to scramble to come up with a way to help my character without messing up our storyline. Eventually one of the mods booted the guy, but for nearly an hour we were stuck dealing with his stupidity.