Just a few off the top of my head.

-Making Roxy drunk type when she is sober

-Believing everything that the worst characters imply about Porrim and assuming that she has sex with everyone instead of having a fairly normal dating life

-Making Kanaya too nice and saccharine and forgetting what she's actually made of.

-Not understanding the difference between "John and Jake are goofy" and "John and Jake are stupid". Neither of them is stupid. Not even a little bit. Oblivious? Maybe, sure, but intelligence comes in many forms, and 'serious smart people' always have blind spots and weak points too.

-This is an antiquated complaint, but it was so annoying when people had Karkat saying "FUCKASS" every five seconds. Fuckass fuckass fuckass!