Oh oh, I have two from AGES ago that I'll never ever forget XD:

One time there was this person in a small group RP that I GMd and they basically went out of their way to get their character together with mine just to magically bring back his dead wife and then have him make a huge thing out of it and go out of their way to try and make her as miserable as possible (such as literally killing off all other characters who had shown interest in her so she HAD to get with this guy just so he could fuck her over a few IC months later)
This same person also would make up random lore on the spot for these characters that everybody else's characters were all supposed to know and respect but the lore things that I had set out AS GM FOR BACKGROUND OF THE RP were shat all over and never followed by them because they didn't want to and it conflicted with their stupid on the spot lore which  they kept changing every couple of posts. Then I was bratty and unreasonable as a GM for finally getting the hell in and trying to tell them to get their shit together or piss off basically T_T and like I'm sorry but I wasn't being unreasonable by that and I'll stand by that because the way they behaved was a joke  (especially since they were supposed to be 5 years OLDER than me, and I was only like...14 or 15 at the time? O_o)

Then another one was a fandom 1x1 I did where the character was such a broody Gary Stu that it was painful. Literally every post they wrote, while it kept to the 1 or 2 paragraph length of Casual we had agreed on in our pre-RP discussion it was LITERALLY two paragraphs of this guy internally monologuing and whining about his tragic back story where his whole family died and his village blamed him for it when it wasn't his fault and exiled him etc. etc. You get the idea XD and they never moved the canon plot forward AT ALL, even though we'd agreed to run it as "canon plot with our OCs added into the adventure". So when I got annoyed and pointed this out to them asking them to actually help row the boat with me they flew off the handle at me acting like I was demanding they write 6+ paragraphs of detail post every half hour and I'm like....dude, no. I asked you to stop writing nothing but "oh woe is me look at the special snowflake me feel sorry for me" in silent brooding that the other characters can't even HEAR to respond to even if I wanted to constantly write "oh grow up and get over yourself damn it" and actually contribute to the plot. XD 

Needless to say I don't roleplay with either of them anymore LOL